trych's post in onStart does not fire on scrollTrigger animation was marked as the answer
Ok, seems I can answer my own question. When I use the doc's onUpdate example in this setup, I get back something like this on page load: 5.11247182388966e-7 1 true. So it seems like after page load the ScrollTrigger animation has already started ever so slightly. I fixed my issue now, by giving the ScrollTrigger a starting value of 1 pixel, then it will fire the onStart callback correctly.