That surprised me because they are planning Office on Apple
Planning? Office for Mac has been around for around 24 years.
Compiling is almost ALWAYS IO bound for a project of any size. Why do you think make -j 4 or make -j 8 makes a noticeable difference on even single core machines? Because the compiler spends most of its time waiting on disk IO, reading and writing all those intermediate files.
From my experience when I toyed around with Gentoo a few years ago, make -j worked best when the number provided was {core count + 1}. Any more than that and it slowed down due to context switching overhead and/or disk head thrashing.
VisualStudio also supports parallel compilers for this exact same reason.
It has severe limitations. It doesn't work when you need to import TLB files, it does not work with incremental rebuilds, and it doesn't play particularly nicely with precompiled headers. The former is required for our project to build at all, and the latter 2 improve productivity more than using multiple threads (YMMV).
It's a naive, domestic operating system without any breeding, but I think you'll be amused by its presumption.