In theory yes. Asteroidos has experimental support for a mainline watch. Most vendors don't upstream their drivers and kernel mods. Also Android drivers use an abstraction layer and a different format to some extentent. So you have to reverse and write your own driver.

So they could run mainline if the vendor or a user bothers to upstream drivers and hardware quirks.

A lot of the vendors don't meet quality expectations of the kernel team and sources are usually for older kernel versions and the code would need changes or refactoring.

The mainlining work is usually done by the community, not necessarily vendors. The relevant pmOS wiki page is https://wiki.postmarketos.org/wiki/Category:Watch but it may be less comprehensive or up-to-date than the AsteroidOS hardware support list. Everyone can help by adding technical info about hardware to the wiki!