I think what they are saying is: Lemmy itself has an option you set on your profile that shows or hides read posts. That'll apply forever no matter which client you use, but you can't even see your own posts that way so it's frustrating.

Then the clients themselves, like sync, might have their own option to hide read that works independently of the Lemmy setting, in which case they should have more control over it.

In my case I have Lemmy set to show read posts but sync set to hide them but like other people have said, it doesn't seem to work perfectly.