EDIT: Still unsure as to the legality of this. IDB is well established, but the fact that everyone has boarded is a different matter. Reading the contract of carriage did not bring any more light to this point. And let's be clear, denying someone and removing someone are very different. At least one source says it does matter (unknown reputability) "This is important because involuntary denied boarding only applies when passengers have been fully checked in (including baggage) and are at the gate at a specified time, typically 30 minutes before scheduled departure." https://thepointsguy.com/2014/12/what-to-do-if-your-flight-i...