Perhaps it's a deliberate simplification for clarity, but that last paragraph seems to me to assume a one-dimensional oversimplification of how things are.

Suppose Frieda would be a great salesperson: she is enthusiastic and upbeat, she has a good memory for names and faces, etc. But her test scores aren't good, and she gets hired as a machinist. How much are those good-salesperson characteristics going to help her impress her colleagues in the shop floor? Suppose Fred has similar test scores and also gets hired as a machinist. He is conscientious, has a lot of tolerance for repetitive work, is dextrous and not very prone to repetitive strain injuries. He turns out to be a first-rate machinist. Do you want to send him off to Sales?

Now, it could be that there are people watching the employees on the shop floor and looking out for ones who (even though they may not be great machinists) would do well in sales, accounting, or whatever. But I rather doubt it, and I suspect that a machinist's work-life doesn't give a lot of opportunities to be noticed as a good candidate for a job in anything far removed from the shop floor.