It's more that Itanium was the one system where it originally shipped, and that for no obvious reasons Intel closed-sourced it at some point. The Itanium proprietary firmware was, IIRC, "SAL" (somewhat related to modern UEFI's PEI layer).
For reference, it was possible to run it on x86 (and even the ICC with EBC was provided!) ~2001, including DUET which I ran from a floppy. There was close to same level of source access as today with TianoCore, though probably with different license.
Then someone at intel got a bright idea to close access to source and until the direction got reversed (which got us TianoCore) it was fully proprietary.
https://web.archive.org/web/20100105051711/http://www.intel....
It was originally called Intel Boot Initiative, IBI.
https://www.afterdawn.com/glossary/term.cfm/intel_boot_initi...
You're right, work started in the late 1990s, but AFAIK nothing shipped until the 21st century: 2001.