That reminds me of paladin tanking back in Vanilla and early TBC. A gimmick tank. It made paladins very unhappy, and finally Blizzard gave us full tanking capabilities.
I would go so far as to say that this glyph is a bad idea. Allowing people to think that they might tank, but not actually letting them tank sounds like a recipe for failure.
In my opinion, if Blizzard wants to go down this path, they should embrace it fully. Make Demonology a proper tanking spec, and the Felguard into a pet which enhances your tanking (as opposed to one that tanks for you). I think it might be kind of cool, and would give warlocks an extra bit of differentiation from mages. Mages and warlocks are a bit too similar, and it might be nice to separate them a bit more.
Of course this means a lot more work for Blizzard, adding another tank spec to balance. Not to mention that existing demonology warlocks might be unhappy that they got turned into tanks.
But if Blizzard doesn't want warlocks to properly tank, they should remove the glyph. Half measures are just going to annoy everyone. The experience of early paladin tanks is evidence of this, and Blizzard should really avoid making the same mistake twice.