The Apocalyptic Defense proposal has been closed as "not a real proposal". This, however, did not stop me from committing to it. Unless I misunderstood the purpose of the close mechanism, this seems like unintended behavior.
This is the intended behavior. Closing a proposal is analogous to closing a question. You can still vote on it and it can be voted to re-open. Eventually, closed proposals will be deleted.
We changed our mind on this. Following / Committing are different than normal voting, so you won't be able to follow / commit to a closed proposal, and a commitment on a closed proposal won't count against your commitment limit.