I'm also not convinced they'd be doing the exact same work if they weren't being paid. Surely they'd be scratching their own itches, not those of the company. I guess for some projects those goals do coincide (e.g. "make GCC produce faster code") but I can't imagine this is true in most cases.
Red Hat hires a lot of people to continue working on projects they are already working on, and so does IBM to a certain extent.
Think about it, from a hiring point of view, OSS is amazing- you're getting someone who is already familiar with the codebase, that you already know for a fact is competent, that can already work with the other developers.
I'm not disputing that. Just because they're working on the same project(s) doesn't mean they're taking the same direction with their efforts as they would have done otherwise. (aside from the fact that they probably have more time to work on it when they're employed vs doing it for free)
Red Hat and IBM employees do not work for free.