Hopefully any US citizens over in the EU are able to get back. 1 day notice is not really sufficient to get pax back.
I don't think you're going to have much if any people making complicated routings to avoid the restriction, except for citizens trying to get home over the next week or so. My guess is the UK was left open to get people back to the US who would otherwise be stranded for a month, and possibly for cargo shipments.
If anyone caught it, the president also said that cargo from the EU is restricted... which will kill a lot of companies dependent upon material suppliers located in the EU if they can't get anything shipped.