It's the platform that's proprietary.

Sun used a lot of open parts, but they had absolute control over the platform. Everything in your solution came from Sun or a Sun partner. Fujitsu doing weird stuff with Japanese mainframes was an interesting fact but irrelevant to 99% of Sun's user base.

Intel/AMD combined with EFI or IBM-compatible BIOS? You can get that from anybody. An HP server is basically interchangeable with a Dell.

Sun was proprietary in the same way Apple is.