I'm not against the choice, and I can imagine some valid reasons to choose it, but in the helloSystem docs I read so far I only found some common criticisms of desktop Linux UI (bloated, unfriendly, disorganized etc). Whether true or not, I really don't see how the kernel choice impacts the graphical experience they are imitating. This seems more of a criticism of distros, poorly followed or non-existent HIG for DEs, or even just the general ad-hoc nature of FOSS projects. That's why I don't see how basing a new free software project on FreeBSD would solve any of that -- it seems completely orthogonal.
I mean, either way it seems they are basing it on KDE, which they could have also built on top of a Linux kernel: https://hellosystem.github.io/docs/developer/architecture.ht...
Also, I get a lot of their criticisms of existing DEs, but it seems they are tossing a lot of work out without a viable replacement with buy-in, e.g. "XDG specifications are considered overly complex but insufficient and should be avoided, but may be acceptable as legacy technology for compatibility reasons". There's plenty of valid criticism of XDG, but until there is any support behind helloSystem, it reminds me a bit of this classic xkcd: https://xkcd.com/927/
All in all I'm actually excited about the project, since regardless of the kernel, I'm sure the DE they are developing will get ports elsewhere (e.g. to mature desktop Linuxes and BSDs), and any effort that helps macOS users jump to FOSS-world is also super appreciated!