The first question is of course whether that's true, since resources don't tend to flex up unless someone outside the project dictates it. But even if it's accurate, it tends to produce the situation The Mythical Man-Month was written to condemn. If the ship date can't move and the scope won't bend on core features, there's a very real risk of finding that extra resources can't fix the timeline problems.

You're right that most organizations have to operate this way, and it's not inherently bad; I've just found that "flexible resources" is often used as a way to avoid admitting that the real third category is "risk of failure".