Seems there is "purging", "venting," and "evacuating."
Some new tanks come evacuated - virtually no air in them. Simply fill them with propane.
Some tanks come with air in them at atmospheric pressure. Some simply fill them while simultaneously bleed a combination of air and propane from the vent. 3-5 refills and the tank would be virtually air free. (Others might take them to a place where they would use a special fitting to evacuate the tank prior to filling.)
I have no idea why a bottle with air in it could not be vented.