I don’t know why 3rd-party widgets would bother with fluent design when the 1st-party offerings (e.g. WPF) don’t. Not even new Edge uses fluent design controls.
WinUI sort of has fluent, but it is extremely inconsistent. When should things be rounded vs squared? It’s completely inconsistent internally.
Even if Qt wanted to go fluent they’d have a hard time finding any concrete examples to work off of. I can’t think of any “fluent” apps that are consistent with themselves, consistent with the design system, and not breaking major rules.