It's not stupid at all to use five J-20s to counter one Rafale (he didn't say that one Rafale could beat five J-20s), and I'll explain why. In the first case, the aircraft are on the ground, and in the second case, they are in flight. However, the Rafale is capable of generating many flight hours, which requires very efficient maintenance and a sufficient stock of spare parts at the aircraft's base. However, the Indians have purchased a ‘Performance Based Logistics’ contract from Dassault, which obliges Dassault to maintain an availability of at least 75%, with penalties if this performance is not achieved. Dassault's internal objective is to maintain an availability of 90% in order to have a safety margin in case of major problems with an aircraft.
The result is that the aircraft can fly a lot: for example, during the Finnish tender, Dassault committed to the Rafale being able to fly 1,000 hours per year on a regular basis and 350 hours per month in ‘surge’ mode. In comparison, an F-35 must go into surge mode to exceed 15 hours of flight time per month and is capped at around 20 hours when used by the Israelis in combat operations with priority global assistance for spare parts.
This is because the maintenance of the F-35 is highly complex, particularly with regard to the maintenance of the airframe's surface, which must remain stealthy. It is not known whether the complexity of the J-20's maintenance is comparable to that of the F-35, but it is certainly more complex than that of the Rafale.
So if China had F-35s, it would produce 15*12 = 180 hours per year compared to 1,000 hours for the Rafale, and it would therefore take 5.56 F-35s to counter one Rafale, and in ‘surge’ mode it would take 350/20 = 17.5 F-35s to counter one Rafale.
Well, it may be a little different with the J-20, but we have orders of magnitude that explain why the IAF commander's statement may be substantiated.