.NET is used all over the place in Microsoft, including many Microsoft Teams services being migrated to .NET. Teams is listed on Microsoft’s .NET customers page.
https://dotnet.microsoft.com/en-us/platform/customers
Teams was originally written before this particular release existed.
The commenter was referring to Microsoft's GUI frameworks on offer in .NET and not .NET in general. Of course .NET is used everywhere at Microsoft. C# is one of the most popular languages in the world.
They were talking about Microsoft constantly re-inventing GUI frameworks and then abandoning them. For example, you bring up Teams, but Teams is actually an Electron app, which gets to the point of the commenter.
I'd also love if maui actually takes off since it does seem pretty neat, but until microsoft uses it for teams or any other of their clients, I'll pass.
They also aren't even targeting linux or web browsers, which doesn't really make this more compelling than Flutter.