Navigators are artificially constructed species that taps into psychic energy in very specific ways for very specific purposes. For most intents I would say they are definitely psykers, just of a very different kind than naturally occurring ones.

Consider it this way; both planes (i.e. jets, propellor planes, etc.) and helicopters are aircraft. They operate because of essentially the same principles (airflow, lift, thrust, etc.). However, the way they take advantage of these forces are very different at all but a very basic level and as a result their respective performance and capabilities are radically different. Both can achieve powered flight but each can do things that the other cannot. Actually, a comparison between birds and helicopters might be more accurate in this case, one is naturally occurring and evolved and the other is purely artificial. Again, the most basic forces and principles involved are similar but their functions are incredibly (almost unrecognizalbly) different and don't really resemble each other (nor is a helicopter particularly intended to resemble or replicate birds).