Yes that's understood. If the bladders weren't vented, but collapsed as emptied there would be no venting for the belly tanks.
Belly tanks would have been sealed at caps, to prevent leaking from above (bladders). But that would only have prevented the pumping of fuel after the bladders emptied. They ran out of gas after what I presume was burning off wing tanks to empty. Or, more precisely, a little more than half of the fuel available from the wing tanks. No more was then available due to failure of the great remainder to pump to the engines. Crimped supply line, crimped vent, something like that....