> My recollection is that Apple simply did not support non-Apple hardware
Quite the opposite I think; what you discovered was that Apple had actually (and quite intentionally - I just googled this and apparently it was done starting with the LC series to lower the cost of entry for the education market) designed their video cards to support VGA output with an adapter. Of course with later PCI Macs you could often plug in and use a PC video card directly (with the appropriate Mac driver). The technical (besides the obvious business) advantage of Apple's proprietary connection was that you didn't need to worry about DIP settings or making sure your monitor could sync up with the desired display settings - it just worked when you plugged it in, and the Mac automatically sensed the resolutions that the monitors supported.
It's quite a bit more complicated to connect, say, an early 2000s ADC monitor to a modern PC. It's also complicated to connect a Thunderbolt monitor (such as the UltraFine 5K) to a non-Thunderbolt PC.
One might suspect that this is just another stupid thing that Apple does to lock in and overcharge its customers, and there are some obvious business incentives for Apple to do so, but there were certainly technical advantages to ADC (vs. DVI) and Thunderbolt (vs. HDMI) notably that they also handled USB/data and power as well as sound and video. Also (IIRC) 5K over HDMI didn't work at the time the LG UltraFine 5K monitor came out, but Apple wanted the 5K resolution (as used in the 27" iMac which had the requisite "retina" pixel density) so they couldn't use HDMI. If you go back to the NeXT PC, Steve Jobs really wanted to have a single cable that connected your monitor to your PC, so it had to handle data and power (I believe) as well as video and sound.