Sure, it's impossible to corroborate at this point because there's no direct evidence of it. It's not like he wrote it in a mailing list that we still have access to. It was (according to what I've read about it) a verbal statement made in response to a question asked of him by someone else. I don't know who the other person is, though perhaps that would be a place to look.
References I've seen have, of course, essentially all pointed back to Kay's claims. I imagine this is insufficient in your eyes, so I won't bother finding them for you.
Arguing "it's reasonable and intellectually honest to reject [the claim that Kay coined the term]" is silly. It's not reasonable, because there's no real reason to suspect the claim to be false in the first place. For 50+ years it has been accepted knowledge that Kay coined the term. Nobody — including people with direct experience on the same teams or with otherwise opposing claims — has stepped forward to dispute this fact in all that time. This would be just like saying "Well I don't think da Vinci really made the Mona Lisa. I mean, all we have is his word for it. Sure, the painting didn't exist before him, and its existence appears to have started with him, and people at the time attribute its existence to him, but for all we know maybe somebody else did it and gave it to him to use as his own!" Sure, it's possible... but it's a silly claim to make (and hence not reasonable).
Your position is not "intellectually honest" because it sincerely looks like you're just trying to be antagonistic. What's the point in arguing that Kay didn't coin the term? Do you have some unsung hero in mind you'd like to promote as the coiner? Or do you just like arguing against commonly-held beliefs for the sake of it? I don't see what you're trying to accomplish.
Two more thoughts:
1. The only way to prove Kay didn't originally coin the term would be to find hard evidence of it used in a similar fashion (i.e., with regard to programming) from prior to 1966 (the time Kay claims he invented the term).
2. If you had such evidence, you would need to prove that Kay had seen it prior to his alleged coinage. In the absence of such proof, the existence of the term prior to Kay's use would be irrelevant. Why? Because the community as a whole has gone off of Kay's claim for the whole time. If somebody else conceived of "object-oriented programming", we didn't get it from them — we got it from Kay.