"The National Firearms Act defines "machinegun" as "any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger." 26 U.S.C. 5845(b). The term also includes "the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of the person." Id.; see 18 U.S.C. 921(a)(23); 27 CFR 478.11, 479.11."
Now if 'single function of the trigger' means just one pull then I do not see a problem - but if ATF counts the pull and release/reset of a trigger as a 'single function' then I can see where there could be a problem.
For $400 it seems a bit prohibitive - I am waiting for the single-celled, spray-tanned, "Three Percent" crowd on Instagram, and other websites, to post pictures of their "sweet Navy SEAL controlled pairs" using this trigger, that will be worth it for entertainment value.
MAC on YT had a cool showcase, seems you can overrun the system, which makes sense. I'm sure it would be pretty fun in a pistol carbine type weapon - but as mentioned, someone will get scared/have their safe space violated/get offended and want to ruin the fun for everyone like with pistol braces.