That's possible, but I think in this case, for a few SX-900 boards, the proper amount of grease was not applied correctly during production. As far as I know, all Yamaha arranger keyboards use grease with rare instances of a problem. So it's not really a band-aid kind of fix, rather a more permanent fix that was overlooked at the factory. It happens.

And to add, the Yamaha dealer repair shop did not hear any noise when he played it, and said it was fine, but he decided to double check it and add grease if necessary. Maybe it's just the way it is, we'll see. But since Yamaha was willing to pay the cost to find out, why not?