Earlier quoted context omitted.
I don't really see it as prioritising your own browser. It's a system level feature and uses the system provided web engine. Microsoft doesn't let you swap out EdgeHTML or Trident for applications that use the native OS web frame. But apps shouldn't be forced to use that web frame is the issue the commission had and the DMA attempts to resolve.
Notably, Android does let users swap out the system webview. So clearly such a thing is possible.
But the system still needed a WebView for when Play Services were not available, so they made a way to toggle between them.