I didn't get the notice but based on how their exposure fee email worked, I think only those affected get the message. So their system probably calculated and saw some accounts will be affected very soon, and only those accounts were notified. Same thing with their "exposure fee" emails. Not everyone got them unless they were affected.
When the going gets tough, they can handle your account via autoliquidation. ES options are plenty liquid, so they aren't taking much risk.