You don’t “successfully foil” an operation and THEN raise threat levels and put on all kinds of extra security procedures.
If you think an operation is about to happen but you don’t know the details and don’t have another way to stop it THEN you raise the threat levels.
But if it is was indeed successfully foiled then those steps would be unnecessary because the threat has already been removed.
There is something else going on here today that has not yet been publicly revealed.
Perhaps it is just that they think they caught some but not all of the people involved. Or perhaps there is more. We will see I guess.
But interesting developments today in any case.