When you serve as fast and with as much bounce as Isner extracts then returns are not really that important!
Take his match against Federer for instance! In the last 3 sets Fed one of the great returners was finding it near impossible to break Isners serve as a result Federer was under pressure every time he served!!
By the end of the match the pressure told and the confidence this gave Isner transferred to his return game as he won the 4th set 6-2 and was climbing into Federer's serve and breaking him and will with some of the biggest return winners you will see!