Certainly the developers knew about it in general, or they wouldn't have put the controls in. My contention is that even they hadn't yet fully internalized just how useful it is. By the time of Quake it's clear that it had gotten to the hardcore, and by now of course it's just part of playing 3D games. But it took time for everyone to realize it. 3D was a pretty big jump. I'd say that the "bug" that strafe + forward was faster than either alone, which was pretty common for a long time in 3D games, reflects that. There's no reason to have that bug or leave it in if you have deeply internalized strafing as a viable option. It happened precisely because it was viewed as an exceptional case rather than the way that almost everyone would be running around all the time.

I'd also observe that biologically, "strafing" is fairly hard, and the idea that you can strafe at the exact same speed you can run forward, safely, for long periods of time, is absurd. What we have instead is faster turning than keyboard controls offer, and more flexibility in general, but not literally the ability to "strafe" at 90 degree angles. It's an adaptation to limited 3D environments. (Strafe + forward gives a more realistic 45 degree angle of shoot to movement. I think in reality that would still be a fairly extreme thing to maintain all the time, but it's closer to realistic. 90 degrees is just absurd.) As it is implausible, it is perhaps not surprising it took a while to become popular.