This claim is very similar to another by Duane Gish that I used to think was a one-of-a-kind, but then years later I recently saw it used again by somebody else, plus there's this "new" one as well.
In a four-page
Impact article on Archaeopteryx, Gish spent three pages regurgitating all their standard claims meant to show conclusively that Archaeopteryx is "100% bird and not one bit reptile." But then at the end of the article he then brought up the claims of two anti-evolutionists that one of Archaeopteryx fossils was a fake, that somebody had added feather impressions to a reptile fossil. Thus Gish triumphantly proclaimed that Archaeopteryx is either "100% bird and not one bit reptile" or it's "100% reptile and not one bit bird", it is
not intermediate between reptiles and birds.
No, it's not mental gymnastics, nor is it really a case that they're not thinking. I think it's that they are trapped in a mental prison of their own making, one whose bars are built of their grossly wrong preconceptions; ironically, rather than seeking to escape their prison, they have to exert all their energy towards holding their prison's bars together. It's not that they're not thinking, but rather that they must constantly guard against the kinds of thinking that would reveal their folly to themselves.
I had a friend at church (UU) who had previously been an extreme fundamentalist. For years, he had to turn a blind eye to everyday observations that contradicted his beliefs, he had to constantly practice self-deception (this is how he himself described it to me). Finally one day, that constantly self-deception had exhausted him, so he took a hard critical look at his beliefs (basically applying the Matt 7:20 test), found them to be wrong, and became an atheist and thorough humanist and a much more spiritual person.
And, no, yet again creationists are not in the least bit trying to create a coherent alternative. Their goal is to discredit science in any way that they can, and that is what they are doing. What is Archaeopteryx exactly? They don't care, just so long as they can claim that it's not intermediate. Echidnas? Same thing.
Creationists are arriving at absurdly false conclusions directly because of their beliefs. Which thus fails the Matt 7:20 test yet again.