How does this extend their walled garden? The kernel doesn’t just accept patches willy nilly.

How does this extend their walled garden? They want to capture Linux/MacOS users with that development.

Enticing developers to use their platform isn’t “extending their walled garden.” In a healthy free market, competition is good. Microsoft is encouraging using Linux, just through a VM instead of dual booting (or even removing Windows).

Vendor lock-in, OTOH, would be a valid criticism. But again, this is not that.

Exactly. All these software companies could probably make their apps run on Linux at a minimal development cost. They just don't want to. Microsoft because they want people to use Windows, Adobe for whatever reason.

but they did afaik? there's Excel online.

The whole 'online' thing is the soft entry into the OS war by Google by the means of diminishing of the role of the OS: the OS is like a bootloader for the browser. Browser takes the responsibility of the OS shell and main applications (mail, drive, docs, chat...). So that's a slightly different topic.

Which is hardly comparable to excel.