This ignores the one piece that was IBM proprietary, the BIOS, that they tried to defend strongly. But back then they didn't have software patents, and Compaq

This ignores the one piece that was IBM proprietary, the BIOS, that they tried to defend strongly. But back then they didn't have software patents, and Compaq was able to prove that their BIOS was a clean room re-implementation. And from that point the cat was out of the bag. Never mind that IBM after that tried to go proprietary with the PS/2, and it bombed badly.