Game idea and game concept are two different things, right?

The terms you quoted from other sites are irrelevant because you can't copyright an idea. In any case, the terms here didn't claim copyright of posts, just a license, which is standard for any forum.

I think that it's a little premature to judge. Wait until the site launches. If these really are experienced game devs creating this site, then I can't imagine it being a pure scam, and in that case I would give them the benefit of the doubt. That is, I believe that the stated intention is true, and that idea creators will get full credit.
They are both ill defined. In general I'd say they are the same except in the mind of specific individuals.