By that same token Luke Skywalker wasn't Vader's son in A New Hope as Kim Bruun pointed out. So yes, this is just like Dr. Pretorius was retroactively Frankenstein's philosophy teacher even if it was never mentioned in the first Frankenstein. It's Universal canon. It's not like we're trying to apply Hammer continuity to the Universal films. Son of Frankenstein states Ygor was in fact working for Frankenstein. What or when we can't be certain unless Universal decides to codify it some more. Obviously this is all in good fun, but you can't pick and choose what is and isn't in Universal lore. I don't think it's unfair for the general public to label Frankenstein's most iconic assistant as Bela Lugosi's Ygor. How about this for an obvious common Horror misconception: Jason does not appear in a hockey mask in Friday the 13th.