The 'own C compiler' is from Joel Spolsky from when he worked at Microsoft: https://www.joelonsoftware.com/2001/10/14/in-defense-of-not-.... And indeed this states that it was p-code. I'm old enough to remember when p-code was - briefly - a very hot idea.
I know we shipped p-code for general usage, not just internal like Excel. But it might have been internal-only in the mid to late 80s. I found a link [1] talking about some p-code internals, from April 1992, by the guy who hired me at Microsoft about a year before that. The compiler team definitely viewed Excel as one of their most important customers, and were willing to do lots of work to satisfy them.
So p-code wasn't actually dead when I started, but instead shipped with the C/C++ 7.0 compiler, which predated Visual C++. I never worked on C7, since that was 16-bit, and p-code wasn't part of any 32-bit compiler, as I recall.
