From what I understood of the article, it's not just the size (which you can get from merging previous black holes), but the combination of size, speed, and angle that are raising eyebrows.

Smash two random black holes together, and the odds are, they're spinning at different random angles. Do that a bunch of times, and unless their angles all happened to be lined up just right, the the resulting spin will be a lot slower than the maximum speed a black hole of that size can spin. But these were spinning at 80% and 90% of their max speed.

Okay, so maybe they were both "normal sized" black holes that gobbled up a lot of matter around a galactic nucleus? That might work, except then you'd expect them to both be spinning in the same direction - but they weren't.

So, none of the scientists' predictions are really matching what they actually observed. Maybe it was one of those things, maybe those models are off a bit, or maybe there's another model to explain these kinds of black holes that we just haven't thought of yet.

When the star is sufficiently big, it will simply generate a smaller black hole at it's center when collapsing, and the rest of the star mass that did not get included into the black hole will suddenly lose the structural support of the star core and will swing inwards and then outwards and form an accretion disk, because it's impossible for all the outer mass to fall exactly at the center, there will always be some rotation and wobbling, and the black hole itself is very dense, so it will consume only as much mass as it's cross-section. So that places the upper limit on the black hole size.

There is no clear theory on how supermassive black holes were created, supposedly the specific region of space just happened to contain a lot of smaller black holes right after Big Bang, and they did not get enough time to fly away and merged, and then slowly fed on gas clouds.

The stellar bodies like stars and black holes are also located very far away from each other and are very unlikely to ever meet, so two black holes colliding usually means that they have somehow traveled at sufficiently close distance from each other and got entangled, then they rotate each other for millions of years just bleeding their kinetic energy as gravitational waves, until finally they merge. So a black hole merging with another black hole is already an improbable event, but we are catching many such events because we are listening to the whole observable universe at the same time. And now imagine the chances of that merged black hole to bump into another black hole.

For the same reason the merged black hole is always rotating, it's just impossible for two black holes to collide exactly center-to-center, any small offset means they will orbit each other, and near collision they will orbit each other really fast.

You can see in the article in small the distribution probability of black hole masses from mergers. It's just very improbable for BHs formed from dying stars to merge into one of a mass of hundreds of suns, but there they give some possible explanations.

In general the important big mass gap in BHs is the one between the ones formed from stars and the supermassive ones, which probably have a different origin.