It was not relevant in TESB, either, since Lucas did not explore any part of a history for Fett on screen (the opposite of what was done for any OT references Kenobi or Vader made about themselves or the past, which was the heart of any PT that would be produced). In 1980 and '83, to the target audience--the movie audience, the majority would never read Glut's novel, action figure backing cards, the ESB poster magazine, Bantha Tracks, or anything else. To this group, he was just "mystery man who hunts Solo, and takes him away" as he was meant to be.
When Lucas decided to make the prequels, there was no on-screen obligation (from anything said or seen in the OT) to even add any Fett-related characters at all. The clone plot was such a separate matter (tied more to knowing what Palpatine was guaranteed to do with them) that anyone could have been the source, so again, Fett (both) seems shoehorned in because Lucas responded to the "cult of Fett" that popped up in the years between the end of the OT and into the 1990s.