Wikipedia says Colt M4 rate of fire is between 700-950 rounds per minute. that works out to 11 to 19 rounds per second. rpm, divide by 60, right? sustained full-auto fire, totally different question. H&K M27 sustained rate of fire listed as 36rpm? I guess that takes into account mag changes.
He specifically said AR-15 in his "15 seconds" ad. So, we have to stay in the semi-auto world.
Bushmaster is one of the few makers that states a spec for their AR-15 rate of fire, and they say 45 rds per minute. That's the effective rate of fire in semi-auto mode.
Arguing over rate of fire is not productive without constraints. First, the cyclical rate is how fast the firearm can mechanically fire, chamber and fire again. That's usually much higher than the operator can effectively aim and be on target, and higher than the firearm was manufactured to withstand the heat from a sustained high rate off fire.
Bump stocks are a "thing," but the bump fire function can be duplicated without the stock. So, it really comes down to the durability of the rifle itself and the shooter's knowledge and time spent practicing.
Realistically, I still think the most common AR-15 has a gas tube, not a piston, and the most common mags beyond the standard 30 rd size are 60 and 100 rds.