Yes, it is certainly a considerable and plausible explanation that what they have experienced is entirely a psychological effect. Tho, what then is the psychological or neurobiological cause of this effect and how might knowledge be obtained regarding such a claim?
As to your question:
How can anyone who hasn't realised the truth claim that Mahatma Buddha or any other person in history realised the God or Truth?
I am not sure what you mean by either "realised the truth" or "realised the Truth".
Truth is simply a condition of statements which is satisfied what what is said is corresponds to (matches, fits) what is (the world, the case, states of affairs, etc.)
Given this condition, it is useful in cases of adjudicating knowledge claims to distinguish what is true (correspondence of utterance and what is empirically verified) from what is "true to [you; me; us; or, them."
While it may be "true to [you; me; us; or them]" that "[you; I; we; they] have been divinely [inspired, contacted, communicated to; revealed to; et cetera]" such claims are epistemically limited to their status as self-knowledge.
For example, a mystic or a lay person may describe an experience as "God spoke to me" and for all intent and purpose this description may be sincere and the experience meaningful to them. It is, however, impossible for the claim to be verified by anyone else (much less falsified - how would you demonstrate that God had not spoken to them?) Such is a distinction of self-knowledge from empirical and axiomatic knowledge claims.
So, to answer your question, "How can anyone who has not X claim that someone else has X" it is worth considering the mundane answer to "how?" that people use their words. Of course, if someone who has not X and has spent a long time trying to X it may be psychologically advantageous to claim that someone else has X.
As for your stated intention to "know the truth" consider that truth is not a statement of what is. That is instead what true statements are. Truth is rather the state or condition of correspondence between what is and what one says is what is. And that is all it is. The phrases "truth of tomorrow", "one truth" and "truth of the physical world" etc. are thus epistemically vacuous. Rationally assessing a statements truth value requires logic as well as sense perception. To know the truth value of a sentence requires the empirical verification of what is said is. Knowledge is in fact empirical verification of what is (else how do you know what is?) Descriptions of what is admit of degrees, but truth value is exclusively either true or false.
For example, the following statements can be empirically verified:
- the moon is roughly 239,000 miles from the Earth.
- the moon is more than a mile away from the Earth.
- the moon is closer to the Earth than the Sun.
All three statements are true. Not a single statement purports the exact distance between the two spheres perigees. All three statements can be falsified and empirically verified. When you empirically verify the statements of what is, you know the content of the statement. And this is the point: the distinction between truth value (exclusively true or false) and knowledge of the statements content.
So, what to make of the claim that "God spoke to me"? The person who makes the claim sincerely and has a memory of the time "God spoke to them" can only describe the case and claim it as "true to them". There is simply no way to also know that "God spoke to them" except if "to know" is used to mean "agree". There is, however, a vast epistemic difference between agreement and verification. Hence, statements of self-knowledge - even mundane statements such as "I feel glad" - are not rationally assessable as either true or false and empirical knowledge may not be claimed in instances of self-knowledge.
