Firstly the R10, R15, and R18 aren’t powered by, “you guessed it” 10, 15 or 18 cylinder engines. It isn’t precedent that Audi’s R cars have the same number of cylinders as the number in the name.
The R8 V10 and V10+ already existed in the first generation R8. So it would make sense for R8 V6 name to make an appearance…