You seem to be deliberately missing the point yet again.
It's not "merely preventing your using it." I am able to prevent your using it because you no longer possess it, I do. I can go for a joy ride in it because it's in my possession. Just because I don't, doesn't mean I can't.
This is the issue. Seizure involves more than the mere denial of use, it involves taking away the possession of that use from someone else and in doing so having the possession yourself (it giving that possession to someone else.)
It doesn't matter if Congress was unable to do something unless they actually lost the power to do it because the terrorists took that power themselves then it isn't a seizure.
Congress still had the ability to vote on the certificates, they never lost it, the Terrorists never gained it. Yes, they were denied the ability to exercise the power because the law says they have to be in a certain place to do so, but they never lost the power itself, any more than going to lunch means that they lose the power.