Taking one alt example to reply.

You are correct, but interestingly enough I noted that most of the instance admins would reach out to me when I create my alt to check if I am actually me.

So it is indeed possible, but there are some light checks.

Also with the amount of posts and comments I made on the feddit.org alt, I would have noticed it early.

I just added a note about the feddit.org alt in my bio to make this more clear.