This is a valid questions, because iirc, as long as these companies don't distribute their linux fork outside of their company, they do not need to release the source. GPL takes effect when you distribute binaries, not when you create them.
Arguably anyone who has a phone, has had a copy in executable form distributed to them, and can ask for the source.
Indeed, if you focus on the preceding bit about Google's in-house linux for their own servers, then there would be no distribution and arguably GPL2 could not force those modifications to be made public.
But I would imagine that he's probably quite versed in GPL2 and knows this -- so since he brought it up specifically, it suggests he was probably thinking at that point more about cases where distribution does happen (e.g. Android, embedded etc).
Perhaps he considers it a 'significant change' primarily if there's some sort of distribution ;)