IIRC Google works the same way. The automated refactoring tools (Rosie?) may make it go faster, but you generally don't fix up 15,000 call sites in a single change... you break it up into smaller batches spread across the tree.
It's been a long time since I was there, but I thought there were plenty of 15,000 call site refactorings done in a single final CL. Not that the Linux kernel should do the same!
When I was there (several years ago) it was rare to do that across many projects. You don't want to have to roll back everyone due to a problem that affects one project. Also, there's a risk that changes that are happening in the meantime might mean the patch doesn't apply.
It may take a lot of time to get all the approvals from all the code owners. One of the big advantages of a tool like Rosie is precisely that it splits up the update into smaller CLs and has automation that nags all necessary reviewers and reruns the transformation once head moves.
