Interesting! My personal experience is that using a vertical mouse very quickly cured wrist and forearm pain that I had, and that the reason is because the "handshake" position is the most neutral hand/forearm posture from an anatomical perspective. But everyone is different, of course
I also agree that it doesn't feel like it makes any sense, hence trying it a few times. I never actually tried going to a medical professional to figure out why though, and thankfully it has not gotten worse (mostly better, with being more careful.)
