Bumping happens before boarding.The contract doesn't allow for bumping after being seated. They're also required to offer up to $1300 cash if you refuse the $800 voucher.
The post I replied to is written as if it is something different than it is (they write as if the contract guarantees a seat on a flight at a particular time; of course it doesn't).
I think it's a bad situation, but I also think people giving very high priority to price is a big part of the reason that the contract is shit.