Which is to say free will concerns “evaporate” if you assume determinism. This is not rationality. Rationality means believing the world is deterministic iff the world is deterministic ,believing it on the basis of a evidence...not assuming it to make problems go away.
The point of the quoted line is not that we assume determinism in order to make free-will concerns go away—as you say, that would be obviously irrational.
Rather, the point is that there are other reasons to assume determinism. (Those reasons are described in the linked post, which is why I linked it.) If, for those other reasons, we do indeed adopt such a deterministic perspective—which, again, there are (or so the linked post claims!) good reasons to do—then we will find that free-will concerns have also evaporated.
(If those other reasons do not convince you, and you think that we shouldn’t adopt determinism, that is fine. You will note that this is not in any way a load-bearing assumption of the post’s argument.)
The point of the quoted line is not that we assume determinism in order to make free-will concerns go away—as you say, that would be obviously irrational.
Rather, the point is that there are other reasons to assume determinism. (Those reasons are described in the linked post, which is why I linked it.) If, for those other reasons, we do indeed adopt such a deterministic perspective—which, again, there are (or so the linked post claims!) good reasons to do—then we will find that free-will concerns have also evaporated.
(If those other reasons do not convince you, and you think that we shouldn’t adopt determinism, that is fine. You will note that this is not in any way a load-bearing assumption of the post’s argument.)
The linked post gives one reason , which is debatable, since it’s been debated. You don’t seem to have complete confidence in it yourself.