Don't get me wrong, this is some absolute BS from MSoft, but no it's not accurate to say you can't use whatever default browser you want. It's that there are a very few select things that force you into opening a link in edge - as far as I'm aware this is functionality mostly limited to just websearch from the start menu. So if you prefer to Google/DuckDuckGo your shit by first opening a browser, this won't really affect you. It's important philosophically and as an indicator of Msoft's behaviour/strategy, but doesn't considerably affect users too much.
Caveat - I haven't used Windows 11 so I'm unaware if they've enhanced the number of places that these edge specific links are used. In windows 10 I haven't even bothered to implement some workaround because I never want to search the web from my start menu