I went and looked at the linked site. I don't find the claim of 1400+ rpm to be credible.
The claim was made by a guy who posted his Utube video on AR15/com.
He describes his firearm as a "Bumpfire SAW" which is obviously just his nickname for it, since it is an AR15 class rifle and NOT the SAW.
He said he calculated the rate of fire, but gives no indication HOW he did that, or what data he used.
Additionally there is mention of an aluminum bolt carrier, so at a minimum, what he's playing with is a highly modified AR15 NOT a standard rifle.
I'm not saying a highly modified AR won't run that fast, but the standard ones, don't.
I's say its kind of like building a funny car off your family sedan. The highly modded car will go really fast, the regular one, won't go that fast.