[quote=@Burning Kitty]
Batman would never help Anarchy no matter what.
[/quote]

If the Joker had info that could take down Lex Luthor, he might work with him on something that didn't hurt anyone, or pay him for it.
That's basically what he's doing here.  He's not convinced that this isn't Anarchy's end game, so he's playing along in hopes of finding out what that is so he can stop it.