"I think we all know what Sig was attempting to do with this "device"."
To be able to sell a non-SBR non-'pistol' version of the MPX action? Unless they've overcome the design limitation*, SIG was unable to get the gun to cycle properly with a full length barrel, which is why something had to be permanently attached to the barrel to get it up to legal length. Since the design was also developed with an integral-ish silencer from the get-go, welding on the monocore from that can was the obvious solution for that problem; gets you to legal length, doesn't push the pressure timing beyond what the design will operate with, and *bonus* you can market it as 'readily convertible' to a silencer with legally-purchased conversion sleeve that wouldn't cost an arm and a leg (well, for a threaded tube it'd still be expensive)
I've not yet seen where 'readily convertible' appears in the silencer text of the NFA, but perhaps I missed it (or perhaps the ATF is assuming things based on machinegun rules that it shouldn't be). If what really matters is whether it drops the sound, the ATF has no leg to stand on; that much is clear. They should have saved their ire for the idiots who would doubtless have started marketing "SIG Solvent Traps" without paperwork, who would actually be selling illegal silencer parts, but no, the had to try for the brass ring. What is not clear, is how judges can continue holding the ATF's arguments as clearly unfounded/indefensible, yet refuse to rule on the case in SIG's favor; frankly, it's starting to get embarrassing --what, are they going to rule on the case like that moron DC judge in the Witashek trial? "I do not know what silencers are, but I am convinced these are silencers. These are silencers, though they do not meet the statute definition for silencers."
It kills me all the folks who think SIG is in the wrong here, but who freely admit to using 'linear compensators' or other muzzle devices specifically to manipulate sound. Sound direction rather than emission, but the intent to quiet the report is certainly more arguable than SIG finding a dual use for its suppressor mono-core outside the NFA. Sorry, but until the ATF rules that threaded barrels are 'silencer parts,' the barrel can't be part of a silencer, so if you weld the core to the barrel making them 'one' it turns into an unregulated barrel. Tough beans, ATF.
TCB
*Clearly not, as it appears there are still nothing but 'pistol' variants for sale. SIG could be making money hand over fist right now with a plain ant-eater-barrel 16" 9mm carbine, the type of firearm 100% of their customers would rather buy, but they do not have the option while this case progresses. If the ATF wins the case, I strongly suspect the MPX to be essentially withdrawn from civilian sales within a year, since *shock* there is no market for factory-made SBRs of any size, thanks to the NFA. Hopefully if SIG loses, they'll bring a suit from that perspective, suing for all their revenue lost at the NFA/ATF's hands.