Lead dev here.

While you're not wrong that we might be under-estimating risk, I'll note that we do take into consideration seroprevalence in our calculations (so called, ascertainment bias). We provide estimates for two cases; 5x and 10x under-counting bias.

And you are right, that risk is additive (to a degree) if you go do multiple parties. And people are terrible at doing this sort of calculation. Our messaging, in our manuscript/press/social media has always been to limit gatherings of any size, and mask and gather outside as much as possible