Well, obviously, there's no way of telling or he'd simply crack the file and say it was random.
His point is based more on softer considerations like "they never used the previous insurance file yet, have they?" and "Wikileaks pulls a lot of stunts" and "this is just the sort of thing that would appeal to a cryptogeek like Assange whose avowed purpose is to throw sand into the gears of secretive organizations" and "so how did they get 300GB+ of leaked stuff since the site has been half-inactive for years and Assange completely distracted by his legal issues? Who would trust them with it? eg. Snowden didn't go to Wikileaks".
If these ever did get released and were damaging, the media/propaganda machines around the targeted governments would be doing dampening/PR anyway. So if that can largely stop the damage, they certainly wouldn't be any good as insurance, whoever releases them.