Reboot means the previous continuity is debunked. That's what it is called "re boot". Everything begins again, perhaps in very different ways.
Predators was not a reboot, it acknowledged the previous movies. It was sorts of a sequel, as it is factually set after Predator and Predator 2. And in commercial terms, a purposed relaunch of the franchise, although it didn't last.
I can simply call it "another Predator movie"; there's really no distinction between Predators and the previous movies, as they share the same basic creature, dynamics and mythology with very little differences (even if the Super Predators were introducers), they are nonetheless set in the same universe and continuity, there's no way to tell apart so it's loyal to the universe of the previous movies.
A reboot would mean none of the previous events would have happened in the movies, or at least they're not referred. But also a reboot is most usually very different (in tone, plot, maybe even mythology) and follows a different route. It is also supposed to kickstart a new continuity/series, not be just a single movie. If it's just a solo movie, it could be called more likely a remake, but it should follow the basic plot of the original movie.
Prometheus is maybe a prequel, and it's also a spin-off movie of the Alien franchise, that is purposed to be a sorts of relaunch of that universe.
