It's probably the same sand-in-his-chamois guy who told me last week to "Cover my effing light", if the trail you refer to is the Sammamish River Trail. Did he cover and uncover (manual blink mode) at you before scolding you?

If you're riding a Giro 20, then your light is already much lower than the average diamond-frame rider who mounts on their handlebars or helmet. You'd have to point the light in an essentially useless fashion (center spot straight forward) for there to be enough top-spill that it's in anyone's eyes.
I have my Supernova E3 (which I know is an extremely bright lamp) mounted under my handlebar bag, below my front front rack, on the drive-side of the bike, and angled down... yet this dude still yelled at me.
As I stated; some people just need to rinse the sand out of their chamois and get on with their life.