RG 59 is fine for joeys, it is all about frequency and voltage. Moca frquencies are under 1,000 MHz, RG59 can handle that fine for any reasonable distance. Also, joeys have no voltage on the line. The only reason the 59 would be replaced if there was a problem, would be because they think the line is bad or damaged somehow, and obviously they wouldn't replace it with more RG-59.
FWIW, every Hopper I have ever installed has been on house wiring (assuming it is RG-6) and I have never had a problem. Just because it isn't rated to 3GHz, doesn't mean it wont carry it fine. If it has been working on DishPro (2.25 GHZ) for any length of time, it will work fine for Hopper.