They distribute on Linux with an AppImage, though, which at least isn't a Snap or Flatpak, so there's that, I guess.
If you want a non-proprietary Electron application, take a look at Logseq, or Athens (YC some year that I'm too lazy to look up).
If you want a non-proprietary non-Electron application, Emacs has a Roam mode, and a couple other Zettelkasten modes.
If you want all that, but you don't care about proprietary operating systems and you want it following the Apple HIG, the popularizer of the Zettelkasten method did it via Textmate.
If you don't mind those things, it's a shiny new application. You might enjoy it. The pricing model is a SaaS, and they lock down the API and limit the amount of entries you can have if you're using the free tier. Standard stuff. It's a very shiny application, and has a dark mode, apparently.