A couple of things, in this forum, questions are usually answered by community members, not by the Itch staff, except for the administrator, who sometimes stops by and answers something.
The only official answer is the one you put in the link, and it implies that it is a bug.
Why hasn't it been fixed yet? I don't know, maybe it's hard to find the error or it only happens to some people or it's not something that's on the staff's priority list.
If you have web development knowledge, and you manage to find the cause, maybe it will help solve the problem.
I just tried with Firefox and I have the same problem and if I disable the paste event of that textbox, I can paste, so it seems to be a problem with the handling of the copy/paste events, but my web development knowledge is too basic to be able to track down the problem.