I have never understood the concern myself, from the standpoint of a split being thrown the way you described it. But here’s the problem. ASMI says there’s no greater danger in throwing curves than FBs, using the same kind of fundamental understanding. As long as the pitch is executed using narrow parameters, there’s no problem.
The problem is, not everyone throws all spits exactly the same, any more than they throw all curve balls exactly the same. All it takes is a minute change to have the pitch go from a benign pitch that helps a pitcher, to a malignant thing that will eventually ravage a pitcher’s arm somehow.
What’s difficult, is to find people who can tell the difference! Sadly though, in many instances the result is all that’s looked at. If the result is good, no one really cares about any dangers that might exist.
There was a time when my boy was around 15, that he suddenly began having issues in his forearm. After he’d thrown for a while, he began to have pain in his forearm, and the only way to relieve it was to throw different pitches. Luckily we had a friend who was an ex-ML pitching coach, and when I told him about it, he didn’t even blink an eye and came up with the answer. “The boy’s holding the ball too tight”.
As it turned out, it was only happening on his FB, and it only took one pen session to get it corrected. Turned out that little change caused all kinds of things to happen. His velocity picked up, his movement picked up, his control improved, but most of all he was happy to be able to pitch pain free, which lifted his spirits too.
There’s no doubt in my mind that there were and are lots of pitchers out there who suffer similar maladies for similar very simple reasons, but how many of them are as lucky as my son was to have access to someone who could help him? Taking that into consideration, its easy for me to see how throwing a splitter could very well be a major reason for an injury, the same way I can see a curve ball thrown improperly doing the same.