Does Microsoft even use WinRT "internally"? Back in the day, e.g. Office would simply ignore all DevDiv and WinDiv bullshit and roll its own frameworks and even _custom controls_ libraries.
So if the designers have drawn up the UI for the next major refresh, Office doesn't need to wait for the shell team to have implemented the controls, or for development to have them wrapped up in MFC.
If the dependencies already exist, you're on safer ground. Though team culture may choose otherwise; I understood Excel didn't like having any external dependencies.
[0] https://www.joelonsoftware.com/2006/06/16/my-first-billg-rev...
Also many WinUI components have come from Office's experience adopting UWP APIs.