WinRT is not the same thing as managed .NET code. There is no requirement that a UWP is .NET. There are many examples of unmanaged C++ UWPs, including the open source Windows Terminal.
WinRT is a mechanism to express APIs in a way that is amenable to cross-language usage. It is built on top of COM, and is not a replacement for COM.
Windows Terminal is sort of a clusterfuck of multiple programs, only one of which is a UWP program (the actual visible WinUI shell that runs) which you're not allowed to do as a UWP-era Microsoft Store app, but they got internal permission to do that.
Its now a WinUI 3.x program, apparently, and now the Store no longer requires UWP programs, so WT now only needs to make the "ships inside of Windows release images" guys happy, which apparently is harder than making the UWP-era Store guys happy.