Actually, both Nest Hubs run on the Chromecast-platform, not on Fuchsia[1]. However their UI is actually made with Flutter[2]. However, both devices (as Sherlock and Astro respectively) exist in the Fuchsia repo and so it's safe to assume that they're developing Fuchsia on them and since Flutter is multi-platform they can replace the OS underneath via software update, if they chose to.

[1] https://www.reddit.com/r/Fuchsia/comments/blyutb/the_nest_hu... (See top comment)

[2] https://twitter.com/timsneath/status/1125868510645669888?s=2...