Not entirely true. Really good engineers may not be good at getting what they are worth and be willing to see what they can get. Finding a job for a higher salary is a lot of work, finding the maximum salary (not that that should be your goal) is very difficult, negotiating is a huge challenge while auctions usually increase prices more easily.

Sure, an excellent engineer employed at $200K with a vast network is unlikely to send his resume to Craigslist ads, but they may use a simple way to find out whether they could get $300K and a $500K signing bonus.