That said, the more flexibility you have the easier it is to find a new position. This naturally favors some people over others.
In my most recent case, it ended up being a bit more challenging due to having to balance the constraints of family life versus the opportunities availed by an extremely arduous commute, but I ended up finding a project I enjoy that works with my commute constraints.
It's worthwhile to compare this relative downtime to ramp-up time for new hires... who also generally spend a couple-few months being fairly useless (though working hard at it).
And googlers being googlers, there's a decent chance that the 'down' time turns into interesting learning or side project time.
Also interviewing with other companies. The next best job for you may not be with Google even if you could easily get an internal transfer role. And teams don't just blindly accept you because you already work at Google (although it helps). You still have to go through a somewhat more informal interview loop that you will definitely want to prepare for.
Of course this is up to the individual, you can definitely just screw around for awhile and apply for an interal role with much less care.
I've dealt with trees in every single domain I've been in, frontend or backend, whether doing "real" programming or building with no-code tools like FileMaker. I've seen admins and accountants build trees and recursion with spreadsheets. I don't think these things are as "gatekeeping" as you imagine.
I don't agree that all jobs have to deal with trees either.
There are other teams where there is little need for more formal process.
I think 3-4 moved within the company, 3 left on their own time before the deadline, and 4 waited out for the redundancy payment.
Edit: Wow, just realized that could be a veiled reference to artificial insemination
I've worked away from the head office at other companies, and it's limiting.
For me the quality of life is so much better outside of SV.
As to people on re-deployment, it did happen that poor performers had a hard time landing a new role -- but then again, they would have had a hard time staying in the company in their old role, so that kind of fits your point. But Intel didn't use redeployment as a "shadow RIF". Intel was not shy about moving people out of the company -- they didn't need to nor bother to hide it.
Back to redeployment -- I can remember occasions of fairly large redeployments (business unit shutting down) where those of us in parts of the company considered more strategic would find ourselves suddenly rich in hiring reqs. Again, the company created the right market conditions and let the hiring managers and employees sort it out.
I guess it helps keeping the workforce young and flexible.