Were you seated while riding downhill? On my gravel bike, I'm always out of saddle on the downhills mainly for balance. Though it probably saves my seat posts from snapping as well.
I'm curious what amount of force is needed to snap a carbon seat post.
From my original picture where I show my snapped seatpost (not the minor cracked one, the one I snapped fully), I can tell you that it's A LOT. And, I'm fairly certain, in hindsight, that one was already cracked a little. I was riding a rough patch of gravel/singletrack levee trail at night and there was a massive concrete step-up and step-down somewhere in the trail. I nailed it at about 16-18mph while on the seat and heard a massive "CRACK". It was so loud that one of the other guys in the group thought HIS seatpost had snapped, lol. I didn't notice how big it was at first, but when I had to turn around and leave the group I rode back over it and saw that it was a solid 6" (or more) concrete bump. I don't remember if it was on the step-up or the step-down, but I think it might have been the step-down that got it. All 180lbs of my fat ass + bike slamming down on it at that speed could not have been easy, and the sound that rang out as it gave up the ghost was impressive.