The anthropic principle isn't providing evidence for the argument, nor is it a universal counterargument. It's just stating that the specific counterargument "well, the world has never ended before" doesn't work.
Its not a counterargument to basically anything, except as to avoid having to deal with actual evidence.
It is an argument that seems almost tailor made to have to ignore mountains of evidence against you.
In any other contexts the supposed "rationalists" would be fully in agreement that having evidence matters, and that not having any works against you.
So, in order to fight against this severe issue with their arguments, they have to invent a reason as for why the entire concept of evidence itself doesn't apply to them and they get to ignore normal evidentiary requirements.