In this scenario, what is a "people manager" supposed to do?
You still need some form of functional management to get the right staff, make sure they have what they need to be able to do their jobs, and deal with day to day issues. This might sound trivial, but it really isn't, and somebody who does this job well will enable a lot more success than somebody who treats it as a minor task.
And you also still need some form of product and project management to ensure that the work being done is aligned with strategy, marketing, sales, finance, etc, and to serve as an interface with those groups. Failure to do this effectively creates massive inefficiencies.
As such, I really don't think any moderately intelligent competent manager is at all concerned by Scrum, Kanban, XP, TDD, Lean, or any other Agile practice.
The good managers are really good at this. The bad managers...well, I don't work in that part of the company.
They're often drawn from the pool of former high-ranking engineers, or at least the good ones are. It's a very different skillset though; part of being a good manager is knowing when to step back and let your reports handle things, and the worst managers are those that haven't managed to separate their engineering ego from their team building ego.
A factory worker on the other hand, can't (legally) take the machines with him.