The way I understand it, the vast majority of the time, DU is readable by people who don't have accounts here. DU threads come up in general Google searches, too.

So if you post a link to something written here on DU, someone can click it and read, and who knows, might end up joining as a registered member. They will also be able to read the public parts of your profile.

But that is different than them being able to post here as if they were you. They only way they could do that is if they sat down at your computer (or phone or tablet) and you were signed in.

A way to check on this is to email yourself a link, open it on a different computer (or log out of your account here), and try to reply. You should be asked to log in or register an account.