And if I'd lost the split second I gain by always covering the levers, this story would probably have a bad ending as well.
At an off-road workshop, our instructors (International Six-Day Enduro and Last Man Standing racers) taught us, "Always keep two fingers on the brake lever, and two fingers on the clutch lever," when riding.
From your description of the incident, you survived because:
1. You maintained a safe interval from the vehicle ahead of you.
2. You developed competence in panic stopping technique from practice.
3. Your fingers, already on the brake lever (and, I assume your foot on the brake pedal) immediately responded.
I've had similar "near-misses," the readiness of the fingers on the levers helped me avert disaster, also. And, I'm not proud of it, but . . . some of these situations were a result of my own lapses (as in, hurrying, and "just this once," not maintaining a safe following interval).
I'll add another safety contributor: Steel-braided front brake line. In my view, the faster, more positive rersponse of the caliper to the master cylinder pressure afforded by the stiffer, braided brake line, translates to a few feet saved in response time. Sometimes, those few feet may be the difference between riding on and a crash.
Back to fingers-on-the-levers-at-all-times; don't know what the Motorcycle Safety Foundation teaches, but--I'm all for the practice.
Again, glad you're safe!