I can't believe posters think WN can bypass IDB rules by refusing to issue BPs in excess of the number of seats on the plane. Under that interpretation WN would never have to pay IDB compensation.

I suspect the security document is treated as the BP in that case. The question seems to be if the OP had the duty to check in with the GA and specifically ask for a real BP prior to T10. OK and the GA might have said something like, wait until you're called.

Sure sounds like IDB. WN accepted checked luggage, gave them a security document and the passengers were at the gate (T-20) waiting for a BP.

Absolutely no reason for the OP not to file a complaint with DOT.

WN (like almost all airlines) overbook. Had the OP checked in at T-24 and gotten a real BP then a different passenger would have been subject to the IDB.

The OP heard an announcement asking for volunteers. The GA told the OP that announcement was an attempt to secure seats for the OP. An overbooked airline asks for volunteers, no one volunteers and a customer with a ticket isn't allowed to board (not given a BP). How is this anything but IDB?