One way to look at it is MAUI isn't more official than Avalonia or Uno, it just happens to be primarily driven by dedicated team from Microsoft. However, it does not mean that Avalonia or Uno are inferior due to just this fact.
EF Core in that regard is very similar - you can easily use Dapper instead because it builds on the same primitives from System.Data. Neither MAUI nor other UI frameworks have access to internals of CoreLib or adjacent packages living in dotnet/runtime.
In fact, having multiple libraries solving a particular task is a sign of healthy ecosystem, too much centralization more often than not tends to be harmful.
EF Core in that regard is very similar - you can easily use Dapper instead because it builds on the same primitives from System.Data. Neither MAUI nor other UI frameworks have access to internals of CoreLib or adjacent packages living in dotnet/runtime.
In fact, having multiple libraries solving a particular task is a sign of healthy ecosystem, too much centralization more often than not tends to be harmful.