Well, I don't know about that. School ratings work a bit differently than almost anything else would, as anything south of -59 would be considered an F. And frankly, giving a movie 5/10 stars doesn't seem to call for an F rating lol.
Using basic math, assuming that there are 13 ratings total (A+,A,A-,B+,B,B-,C+,C,C-,D+,D,D-,F) that would mean each grade is worth around 0.76 stars. So with that in mind, a C would be worth around 4.6/10 or to estimate 5/10.
Of course - this is all completely subjective as everyone has their own rating system and I myself have never seen this man's reviews. Maybe he clarifies that C=7/10, but in terms of people I know personally I could never imagine them saying "It was alright... I'd give it maybe a C. Definitely 7/10 stars" lol.
That said, I don't care about ratings in slightest and still look forward to catching my screening this Friday.