The snap doesn't make any modications to default user settings (for better and worse), So I apologise if it really is the case that the manual selection of xdg-open is needed for the Flatpak, but I do genuinely believe the default of xdg-open is actually built into Electron itself.

I suppose the semantics here don't really matter, I do agree with you that it should be default, it's just that I'd struggle to explain why it isn't. I'd have to assume that it's due to some nuance in the Flatpak environment.