when Microsoft talks about compatibility they’re largely talking about intranet sites inside corporations, which are often maintained poorly or not at all, so anything that’s broken by a browser change will remain broken forever. And it’s easy to see how the needs of large corporate customers can come to dominate the thinking of a software organization
this is why microsoft utterly fails on the web: they are internally divided, provide bogus arguments against the evolution of the web because it would hurt their cash cows.