It certainly might have been perceived that way. IIRC, Font-lock mode included logic to defer the heavier execution of fontification until a certain amount of i

It certainly might have been perceived that way. IIRC, Font-lock mode included logic to defer the heavier execution of fontification until a certain amount of idle time accrued (particularly helpful from a UX perspective as people are typing there can often be dramatic shifts in fontification, and it isn't worth slowing them down just to make a change that will disappear at the next keystroke). There are also tricks in it to do partial fontification. I believe this is now handled in a sub-module called jit-lock-mode (Just-in-time Lock Mode). However, font-lock-mode never executed in a separate preemptive thread.