If you copy or distribute the derivate work in either object code or executable form, you are obliged to make the source available.

Arguably anyone who has a phone, has had a copy in executable form distributed to them, and can ask for the source.

He was talking about Google's private fork of Linux for their servers to run Google search.

He was talking about companies in general tweaking linux for their particular uses.

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 ;)

Exactly. I guess the license doesn't really matter a lot in this case, as long as it's some sort of Open Source license. The benefit the companies have by getting their patches merged back in is that they don't necessarily have to maintain the patches themselves anymore. The GPL, however, should not have any effect on that, as long as the work is used internally only.

It used to be the case that licenses required you to submit your modifications back to the source. Lucent's first stab at opening Plan 9 From Bell Labs back in '99 had such a clause.