You should read the notes, they help answer your questions.

Anyway, 2 points:

1. This is not about margin accounts. When trading on margin, it's RH that's financing you and taking on the risk. That's separate from what the clearing house does. Incidentally, RH did increase margin requirements due to the higher volatility.

2. The problem is RH offers instant settlement. That is, you can use your proceeds from a trade immediately in the apo, before even the trade is settled. That exposes RH, and the clearing house to settlement failures, and that's true regardless if the trade was done on a margin account or not. And that's the risk that clearing houses try to mitigate via asking for collateral.