I thought the strategy was not to tell them?

Generally, yes, because hell-banning is for trolls, bots, and those who routinely make bad posts. However, sometimes someone gets hell-banned for one extremely down-voted comment, but in general are making decent comments. It's a common courtesy to inform these people if you run across it.

What does it mean to be hellbanned and why can we see the posts anyway?

There's a user setting to view dead (banned) posts. If you see them then you have yours enabled. But nobody can reply to a dead post.