Here is the full context of the usage of under the cut you’re asking about:
For this roundup, we focused on fiction titles where books play a major role in the plot. From there, we surfaced the most popular titles with at least 100,000 ratings on Goodreads. So fan favorites with a lower number of overall ratings like The Eyre Affair, The Library at Mount Char, and The Book of Lost Things are under the cut. You can add to our list by sharing your favorite books about books in the comments.
Under the cut and its allied make the cut come from sports jargon and refer to a score set as a cut-off line which will eliminate a player or team from competition. With timed sports or golf, where the lowest number is the winning score, under the cut is a good thing, meaning the competitor will be allowed to compete further. In this case, under the cut and make the cut are synonymous, for instance, in this news article about the recent Master’s Golf Tournament:
Tiger Woods (+4) struggled again Friday, as did Phil Mickelson, who shot 79. But both did enough to make the cut at five under.
He overcompensated on his chip up, leaving himself a testy 20-footer straight downhill. He drained it to stay under the cut line, which is currently at +5.
If a writer has a leaderboard in mind, where the golfer with the least score is listed first, then Tiger Woods is above the cut, an expression used far more frequently.
And for timed sports like swimming:
How far under the AA-B cut would she have to be to be assured that she would ultimately become an All-America?
For sports like hockey where a high score wins, however, under the cut means that the player or team didn’t make the cut and is thus eliminated:
It didn't finish on the ice, as the Gophers [UMinn hockey team] were eliminated by a mind-boggling combination of six conference tournament results that went against them and dropped them just under the cut line for a tournament berth in the PairWise rankings.
If ratings on Goodreads were a competitive sport, then the book with the highest rating “wins.” Since the cut-off point is 100,000, books that came in under the cut are eliminated from consideration and will not appear on the list of most popular titles.