Aristotle's views on artifacts are a little bit hard for us to accept in the contemporary era, but the basic idea is that a substance is unified by a single form which is its activity and the thing that organizes it.
So I think it's slightly off to say
Aristotle assumed humans, animals and even plants to be substances. On Aristotle's picture, this is not an assumption -- it's an observation from empirical data. To give a couple of examples by differentiation:
A plastic plant and a living plant differ in two significant regards. First, if I snap a branch off the plastic plant, it forever remains damaged in that way. Conversely, if I snap a branch off a living plant, then the plant will either suffer a catastrophic death and stop being a plant (i.e. die) or it will adapt and recover around that and continue its life. In other words, the plant-like artifact just takes damage, but there's no life to take away from it. Second, the real plant is growing and to keeping being a plant needs soil, light, and air.
A taxidermed wolf vs. a dog. We have the same image of damage vs. injury. In addition, the real living animal moves around under its own utility whereas the taxidermed one can't do that. Even if we stick something in it and let it be remote-controlled, then that's not really the taxidermed thing moving around on its own.
To put it another way, a substance has one thing that makes it tick and that orients the rest of what it's doing. And when we break that, for living things we have to kill them, and for other things they completely lose their unifying coherence.
I raise this because the point isn't that Aristotle is pulling a random assumption out of thin air and holding to an irrational faith in substances. At least on his own view, he thinks he's using his mind to intuit forms from living things (and some non-living things).
One contrastive category is the "aggregate" or the "pile." If we think of a mound of sand, it doesn't really have anything that 'unifies' it in a deep sense. There's nothing that keeps the pieces of sand grouped except that they are pieces of sand.
If we think about a CPU, it's really a very well organized aggregate. It's billions of little tiny transistor gates that we've piped together to work out as a super abacus. But it's still a bunch of parts that don't have a fundamental unity or maintain themselves. They have a very useful unity in that we put this tool to good use, but the unity is not fundamental to the thing.
In other words, I deny that a CPU is a "unified material object". The key is to grasp that what is meant her by unity is not "to be in one piece" but rather to be one coherent whole where the parts necessarily compose the whole.
Now, the article Mauro mentions raises some similar objections to what you're raising but does so in the context of a good understanding of the Aristotelian vocabulary and how it works.
If we want to lend credence to your idea, a better example might be an android of some sort. Here, it seems to have internal motion and coherence around its android existence. In other words, it would be better approaching that unity even with non biological parts.
(ps in passing I do want to mention that this is a good question despite my disagreement with your claim -- in large part because Aristotle's view and claim are hard to understand).