Do you really think those are actually deleted from those forums? They might be hidden they might remove your name but it's unlikely they are actually deleted.

Also you failed to understand one basic thing, their terms of service wouldn't list something like your account can never be deleted. Because it's not your account it is their system their platform and what you call an account is simply a way for you to access it and post on their system. The terms of service would have to say something like you continue to remain in control of everything that you've written. It's simply doesn't do that and I don't know of any system that does that. Because it's not yours.