When David Anders said That, Frankenstein was not a very popular guess at all.
For me, it was, but then I spend a lot of time reading various OUaT forums looking for spoilers. I know at TWoP when Dr. Whale was first introduced, someone already came up with the theory that he was Dr. Frankenstein, and I saw it increasingly more popular as time went by especially after the spoilers leaked of David Anders running around with a cooler and that special green thing on his arm.