We have also seen this with the metal hero sets where we get 4 non mounted Elves/Undead/Dwarves vs 3 models in the Abyssal/Orc set on account of the Gore lord/Half Breed Champion. The gore riders will probably be restic but I can still see them being a little bit more expensive.
They are definitely restic, the advance order page on Mantic's website confirms it. You're right though, they do look large and could well be a bit more expensive than the revenant knights.
Honestly, they could have advertised the deals without a limited edition mage and they would still be good value and people would still buy them. In this case I don't specifically see it as a sneaky marketing tactic, and have no problem with it.