The term "prime rib" has nothing to do with the grade of the carcass.
Even the USDA allows the term "prime rib" for any grade of carcass.
It is a marketing ploy. Any reference to the term "prime" in the name only loosely means that it comes from what some consider a "prime" area of the carcass.
Having said that, yes, it does come from the rib section where roasts are often more correctly called a "standing rib roast" which denotes the section of the carcass it is cut from. Rib numbers and all that, but they do matter.
It will still be a good cut of meat, but a good cut of meat within the actual grade of the carcass. In these days that means "select", "choice", or actual "prime" carcass grades.
Supermarket meat is usually "select"grade unless they proudly let you know it is "choice" or the rare supermarket "prime" grade.
But the grading comes from the carcass rather than the name of the cut on the package.
In any case it is a good cut of meat from that particular grade of carcass, and I would buy it at that price and enjoy it!
Good luck and good smoking.