Well, I could certainly be misunderstanding...
Here is the relevant section from the announcement I received:
50% margin close-out on account level basis
Positions will close out at 50% of initial margin requirement to reduce the risk of loss.
Example: Say you have £2000 in your account, you then open a GER30 position that requires £1,000 margin. 50% of the initial margin requirement for the GER30 position is £500. If your equity falls from your original £2000 to £500 or less, then we will close out your positions.
To me, this reads like Margin Call now happens at 50% of (aggregate) initial margin requirement, not $0...
Thanks.