By modern standards this is pretty bad, and it boils down to an exposition problem.

The author needs to explain certain basic information up front (or at least pretty early on). A good way to do this is to have one character be a novice who needs to be told basic details, thereby informing the audience. In fact the "new guy" angle to exposition delivery is so good that it itself is becoming cliche.

In the example you brought up the author wanted to take advantage of the "new guy" trope but for whatever could not do that. Maybe the character needed social status or standing that a rookie would not have in order to make the plot work. Rather than find a creative workaround that made sense in their story they pulled out the old amnesia trick to eat their cake and have it, too.

One book that did it well was Nine Princes in Amber. It worked because the readers got to discover the "real world" along with the main character. Without it there would've been a shit ton of exposition of a detailed setting that didn't rely on Tolkien at all, one that the MC was already familiar with. Although it might have been fresh in 1970 and overdone since then.