With regard to Rabbi Yehuda Hallevi's criticism of Aristotle, I doubt that he is writing it specifically with Timaeus in mind. He did know it (he names it in 4:25), but from what I understand, the Arabic-speaking world knew Timaeus from translations of Galen's summaries of it; I am not sure whether this particular passage would have made it through the process. Even if so, this remark is natural enough in a Jewish context (compare Maimonides' similar comment in Guide for the Perplexed 2:24, end) that dependence on Plato seems unlikely, even if the ideas themselves are compatible. In other words, his statements are not interpretations of Plato, so they have to be compared from the outside.
Comparing him with Plato, there are some important differences between their views: Rabbi Yehuda Hallevi wrote with a specific tradition in mind (the Torah) that he claims Aristotle didn't have access to. If only Aristotle knew the Torah, he says, he would have been able to apply himself to proving true propositions instead of guessing. On the other hand, Plato seems not to expect that any human being could possibly know the answers to such things as the creation:
Wherefore, Socrates, if in our treatment of a great host of matters regarding the Gods and the generation of the Universe we prove unable to give accounts that are always in all respects self-consistent and perfectly exact, be not thou surprised; rather we should be content if we can furnish accounts that are inferior to none in likelihood, remembering that both I who speak and you who judge are but human creatures, so that it becomes us to accept the likely account of these matters and forbear to search beyond it. (Timaeus 29c-d)
The story of the Egyptian priest that Plato brings earlier would initially seem to indicate that he thought that valued the Egyptians' ancient traditions, but in fact, the Egyptian story might allude to another of Plato's Egyptian stories. In Phaedrus, Socrates recounts a story from Egypt, which Phaedrus disbelieves, but eventually they both agree that the provenance of true knowledge is of no value:
Socrates, you easily make up stories of Egypt or any country you please.
They used to say, my friend, that the words of the oak in the holy place of Zeus at Dodona were the first prophetic utterances. The people of that time, not being so wise as you young folks, were content in their simplicity to hear an oak or a rock, provided only it spoke the truth; but to you, perhaps, it makes a difference who the speaker is and where he comes from, for you do not consider only whether his words are true or not.
Your rebuke is just; and I think the Theban is right in what he says about letters. (Phaedrus 275b-c)
In other words, the Egyptian priest's remark against the Greeks in Timaeus seems to reflect an attitude that contradicts Plato's own views!
And there is another reason not to take the entire latter part of Timaeus as vain speculation: Plato's dialogues often end with Socrates saying that he is no wiser than when he started, yet here Plato dedicates the majority of a lengthy dialogue to describing the creation in detail, surely not for no reason, without any similar disclaimer at the end of it. So that even if Plato acknowledges that his creation might not literally be true, he did believe it was useful to go into in detail: there are many themes (e.g. the behavior of the Good) that are certainly not presented as if they are nonsense.
Many of the early Academics didn't take the creation narrative described by Timaeus as a historical truth, but their concern was likely to defend Plato against Aristotle (who did take the creation in Timaeus literally) by making Plato compatible with Aristotle's eternal world (Tarrant, Plato's First Interpreters 44-46).
But even if not to be taken literally, and even if not even presented as a certain truth (as implied in 29c-d, quoted above), I don't think it's tenable to consider Timaeus to be meant as a magnificent display of ignorance (I write in my opinion, not a scholarly interpretation as you were looking for; but then again, sometimes even oaks and rocks can speak the truth). The accusation of a lack of an ancient tradition is significant, as it applies to the creation no less than Atlantis. Yet from Socrates' remarks to Phaedrus, we might infer that Plato didn't feel a need for an ancient tradition. And as evidenced by the fact that the dialogue continues, the lack of a tradition didn't deter Plato from making a serious effort to come up with a reasonable creation narrative.
On the other hand, Plato has Timaeus say two interesting remarks that might imply the contrary:
Concerning the other divinities, to discover and declare their origin is too great a task for us, and we must trust to those who have declared it aforetime, they being, as they affirmed, descendants of gods and knowing well, no doubt, their own forefathers. (40d)
but the principles which are still higher than these are known only to God and the man who is dear to God. (53d)
It's hard to say what exactly Plato is alluding to here. The Loeb editor thinks the first remark is "obviously ironical." Obviously the bulk of the dialogue wasn't written as vain speculation with no claim on truth. But privileged divine knowledge does exist in Plato (as in his metaphor of the magnet in Ion, which would be a fitting portrayal of a tradition), so maybe Plato did in fact portray Timaeus as thinking that the Egyptians' tradition brought them closer to the creation of the world than the Greeks could get. If this was implied, was it meant to make the reader despair of ever getting accurate knowledge about the first principles, or to imply that the reader might get access to a divine tradition at the Academy, or just as an artistic touch? I don't know. But if Rabbi Yehuda Hallevi had seen one or more of these statements in translation, there might be reason to posit some dependence on the dialogue when he makes his remarks on tradition.