Just a theory. But I think they are preparing for the notion that there's a growing number of PWAs in their app stores. And most of those would be using things like maplibre or Google Maps. So, to address that (given that they can't really stop PWAs), it makes sense to make Apple Maps usable outside of the Apple platforms. This way, people can develop PWA apps and have some level of integration with Apple maps on IOS. Just a theory.
The browser restrictions are probably because developing hardware accelerated map rendering engines for the web is a bit of a project and the support for things like WASM and Web GPU in Safari is probably requiring dealing with some Apple specific quirks. Maybe they'll get around to that eventually. I think for most web developers, no Firefox support would be a show stopper. There's no point to this strategy unless they address that.
They already had MapKit.js as a mapping SDK available across various browsers, off their own hardware. It's been available on a variety of browsers, even Firefox, this is just a beta that doesn't support it.
The browser restrictions are probably because developing hardware accelerated map rendering engines for the web is a bit of a project and the support for things like WASM and Web GPU in Safari is probably requiring dealing with some Apple specific quirks. Maybe they'll get around to that eventually. I think for most web developers, no Firefox support would be a show stopper. There's no point to this strategy unless they address that.