[zokier]

No. GPL doesn't mean that derivative works source needs to be available. Thus Google may very well have modifications which are never made public.

[Rusky]

The source must be available to those who have access to the program. If Google ever distributed those modifications, they would have to distribute the source as well.

[zokier]

That's the big if. Especially when the paragraph was discussing Googles performance tweaks to their in-house kernel used in their servers.