[Rusky]

Derivative works of the kernel must also be licensed under the GPL, meaning their source code must be available. So, if the Linux developers like the changes someone else has made, they can merge them in.

[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.