So the mark price system of BitMEX worked in this case, but it caused lots of liquidations which ended up not being filled and caused socialised losses when the price went up to $460s.
Perhaps with the current price environment and liquidity on BitMEX these DPE's will become a more regular occurance. Or maybe 100x is a little too high leverage to be offering if it becomes really frequent.
Traders have differing opinions on this but most support socialised losses in exchange for safe access to high leverage. It just gets more difficult to defend the more times it occurs especially when more than 1/3 of profits are being taken in events like this.