If the turbo seized because of a piece of a valve, it either would have been visible when the turbo was removed during the engine rebuild (which is probaby not the case, as the description would have said explicitly that the turbo has a piece of valve stuck (somehow) on one of the wheels instead of, "I'm guessing [this is the case].") or it was a piece that got in the oil, in which case it was a jackass move to put the turbo back on the car, as it would have broken metal fragments in the oil lines and center cartridge.
Perhaps I'm missing something, though.