Sounds plausible. The obvious Q though is - were the researchers able to identify a causal effect? Or could it be that the highly-paid new hire is a sign of the market being hot, so the top performers are able to get new better offers? i.e. would they have left even without the new hire?
Agreed they don’t really address threats to validity (HBR almost never does). Another objection - they claim that adjusting salary sooner rather than later has a big effect on turnover, but do they control for the general effect that getting a raise or not affects turnover rate, independent of whether someone was hired at a higher salary?