I suspect there is something happening statistically allowing periodic exposure by air transmission,...not necessarily airborne.
Like the pig-monkey experiment in Canada a few years back where not all monkeys became infected concurrently, but half of them did acquire the transmission from the isolated piglets, and later the others acquired it, again without fomite or direct contact.
Maybe its a blossoming unencapsulated virions, only happening periodically, infecting some percentage.
When exposed to higher concentrations, such as a corpse of obvious contagious stages, then without PPE infection is near certain without antibodies. Meanwhile, not everybody gets it immediately if temporarily exposed.
I agree it is not airborne in the definitive sense- but as you mentioned the monkey study it seems likely or possible that droplets could be inhaled- particularly in late stage when virus is ripe and plentiful.