So when you do your "research" on google (rather than having had any actual technical experience with the thing itself) you're quite likely to get real confused about which part played the role of the compiler (the p-code generator) and which part played the role of the operating system (the UCSD Pascal system-- an operating system written in pascal, NOT a compiler!)
And yes, the Lisa and Mac OS were bad hacks of the UCSD Pascal OPERATING SYSTEM, which was, yes, compiled with a different compiler than the original p-code generator. They didn't want to use the p-code system, because otherwise people would be able to run the derived Lisa and Mac OS's on, um, Modcomps and Teraks! By just copying the p-code to a different machine. That was why the p-code system was so great, it was like Java WAAAAY before its time.
But to think that the p-code system was the SAME THING as UCSD Pascal is just as stupid as thinking C is the SAME THING as Unix.