AsteroidOS mostly supports watches that come stock with Wear OS, which is a modified version of Android, which, guess what, runs a Linux kernel. These watches are on the more powerful end of the computing spectrum. As you say, there are a lot of smart watches that use pretty lean MCUs, but those aren’t running AsteroidOS or Wear OS, as noted in this FAQ entry.
AsteroidOS mostly supports watches that come stock with Wear OS, which is a modified version of Android, which, guess what, runs a Linux kernel. These watches are on the more powerful end of the computing spectrum. As you say, there are a lot of smart watches that use pretty lean MCUs, but those aren’t running AsteroidOS or Wear OS, as noted in this FAQ entry.