Which strikes did you assume? I'm wondering if most of their loss is due to convexity, as RedDuke mentioned the "insane" prices we both observed between Feb 5 and 6.
It is one thing to sell a put for $100 and see it go to $400, but it is a little different if you sell 50 puts for $2 and they go to $50 and you can't delta hedge on the way down, which I believe is something they wrote in their recent letter. As spot moves down, these $2 puts have more and more vega and your delta hedge won't help much.
My guess is that they had excessive leverage for implied vols they did not anticipate.