Author here. Great point, and I think this is due to what another commenter points out, that the questions are different.

The right test of this is to take the _same_ markets that run for 90+ days, and check accuracy 90 days out vs 30 days out. I've done this on other prediction market datasets, though not on Kalshi and Polymarket, and found that forecasts are in fact more accurate 30 days out.

I agree that if they weren't, that would be incredibly suspicious!