I don't see why it's surprising that they'd consider the umber hulk a WotC trademark, though. It's just as much an original D&D monster as the beholder and the mind flayer.
It's not functionally a trade mark as it's never been used as a badge of origin. But they could conceivably use it in the future, as Beholder arguably was with Eye of the Beholder game. A book or game title is not necessarily a badge of origin > un/registered trade mark, but it could be.
Anyway, it still doesn't make much sense to use TM law to sue someone for use of SRD material.