While this is true... it's also letting yourself be massively taken advantage of, and underpaid.
Yes, the best way to get promoted is to do work above your level. The problem is, you're not getting paid what you deserve for that. If you're always doing this, you're always being underpaid by a full level.
Which is why much better advice is to try to get promoted by switching companies and jumping a level in the process.
Managers certainly want to take advantage of you by getting you to overperform without being overpaid. But employees should do everything they can not to fall for it. Which usually requires getting companies to compete over you.
Yes, the best way to get promoted is to do work above your level. The problem is, you're not getting paid what you deserve for that. If you're always doing this, you're always being underpaid by a full level.
Which is why much better advice is to try to get promoted by switching companies and jumping a level in the process.
Managers certainly want to take advantage of you by getting you to overperform without being overpaid. But employees should do everything they can not to fall for it. Which usually requires getting companies to compete over you.