"IBM had no moat around the PC." is true about the original PC and XT, but they did have 7 patents on the AT. After the PS/2 attempt didn't work out, their lawyers spent the first half of the 1990s going after the chipset makers and the second half of the 1990s getting clone makers themselves to license these patents (along with some other unrelated ones that weren't about to expire).

A quick search showed it isn't easy to find online version of these patents (because IBM has so many that even knowing these are from 1984 didn't help), but I remember that one was related to being able to split a screen into a graphics and a text part in their EGA board (though the Apple II previously did this too, but with a fixed split), one was about detecting 360KB vs 1.2MB floppy drives by seeking to track 60 and then stepping back 59 tracks and checking if we were now at track 0 (not unlike how the Apple II handled the lack of a track 0 signal, but for a different purpose), one was for the "bus master" signal in the PC AT (later ISA) bus and I can't seem to remember the other four but they were all similar in style.

So in the late 1990s you had to pay IBM if you wanted to make a PC clone (AT and up, but 8088 clones had died out in the early 1990s).