Does this source tree include the old Win32 version? Have they open sourced that, if not?
Initial commit in this source tree is from January: https://github.com/microsoft/calculator/commit/ac14e2df10de7...
The major code dump followed alongside the open source announcement:
https://github.com/microsoft/calculator/commit/c13b8a099eea1...
Given the huge size of the Windows repository it probably wasn't worth the effort to recreate the deeper history of the code (and I'd assume probably would have taken a lot of scrubbing/redaction clean up to make everyone, especially the corporate lawyers, happy).
They don't seem inclined to open source the Win32 UI, but it could potentially happen. Maybe when it hits some anniversary like WINFILE.EXE did: https://github.com/microsoft/winfile