I agree that declaring a win is a bit impolite _if_ the other person hasn't agreed. But changing "farmer" to "robot farmer" because Google won't allow him to generate pictures with humans is obviously not changing the goalposts in the usual meaning of the term.
Claiming the generated art is an image of a robot farmer because it's wearing a little hat is definitely changing the goalposts.
1. it's disputed
2. the assertion is that he has "common tendency to shift the goalposts"
The emphasis on common is mine.
I think changing the terms of the bet is definitely shifting the goalpost, even if not by much. It is certainly enough for the other party to refuse the win.
I meant that the tendency is common in techo-optimists, not that Scott Alexander commonly exhibits it. Sorry for the ambiguity.