@garyackerman
I doubt that anyone seriously making this claim hasn't thought about that issue. (Admittedly the "seriously" there makes this a bit of a No True Scotsman!)
I may disagree with their opinion on the subject, but I don't think their error is as simple as not having thought about how slippery the concept is.
I think a typical response would be along the lines of "No, I can't define it exactly, but we routinely ascribe consciousness to each other on the basis of evidence that's relevantly indistinguishable from the evidence that I have about ChatGPT / Bard / whatever, so that ascription is equally correct."
(Also something seems to have gone wrong with your sentence 2 of 3 there; or at least I don't get it.)