I think so. Heck, why don’t they open source it now? Although my guess is it’s a lot of low level C++ that I wouldn’t touch with a 10 foot pole. But I’ve been surprised before. What if they used dotnet?
I suspect it shared quite a bit of code with the regular windows codebase, so open sourcing it would have exposed a lot of proprietary code (and not necessarily only their own — there may have been licensed bits that they would not even have been allowed to open source).