I kind of think that's an even weaker claim by them, since the licenses they granted include the right & duty to re-license all OGC on the same terms. If the old OGL 1.0a licences are valid, then that term is valid, too!
Edit: They're not claiming to revoke a perpetual licence. They're claiming to alter the terms of the licence post acceptance. That seems an obvious no-no.
I disagree, and I view it more in terms of using the contractual language not to revoke a license, but to revoke the offer of the license by no longer offering a license others can use. It's a notable, and salient difference- and one supported in cases I've seen.
What I do know is this- there has been substantial review of this by legal; given that we do not have access to what they have, I tend to think that they have developed something colorable. It's not like Hasbro doesn't pay for good IP attorneys.