I'm not too sure what drove Santiago to change the way he described that pitch. But a lot of guys throwing changeups nowadays are essentially throwing screwballs. Hector gripped it like a circle changeup and threw it like one. I find that in youth ball, it isn't often taught the screwball way so a lot of fans don't realize this. But the best changeups are thrown with total wrist pronation, basically like you'd throw a curveball but with your hand facing the opposite direction (and usually gripped differently). This generates side/overspin, but of course due to the fact it's generally harder to pronate than supinate, it comes out harder and with less overspin than a curveball. Some guys can't really do it at all but they tend to have an easier time throwing sliders. Other guys who are natural at spinning the screwgie changeups often can't throw a slider very well. The "changeup" Devin Williams throws is very much an all-time great screwball.
As for why fewer curveballs, I think the biggest thing is that "tunneling" is the prevailing theory about pitch mixing nowadays. You want all your pitches to look the same not only during your throwing motion but as long as possible after release too. The idea is that they are all traveling down a common "tunnel" towards home plate until at some late stage they diverge due to different movements. The classic tunneling trio is the four-seam/sinker/cutter. These are all very similar out of hand but the cutter darts gloveside late, the sinker darts armside late, and the four seamer stays a bit straighter and will feel like it's rising relative to the other two. Sliders don't tunnel quite as well but the extra movement is a good tradeoff for a lot of pitchers. Same for changeups which diverge a little earlier (mostly due to the speed difference). Curveballs generally don't tunnel very well because the ball comes up out of the hand, making it look different from the heater and everything else almost immediately to a discerning hitter.
One thing I haven't heard an explanation for yet is why it seems en vogue nowadays to attack opposite-handed hitters with curveballs — and not so much with sliders. As best as I can tell, for a long time there has been a reverse platoon split for curveballs (same-handed batters hit them better than oppo-handed batters which goes against conventional wisdom). But this is mostly isolated to early in the count and when the curveball is thrown outside to the oppo-handed hitters. So basically, especially early in counts oppo-handed batters have a hard time judging the backdoor curveball and take it for strikes. Later in counts, they are more aggressive and don't take those pitches and don't whiff on them either. Same-handed hitters have an easier time judging curveballs because they can't be back-doored and the break is so big and slow they have a chance to measure it in a way that is unlike the slider.