You have to think about the sound of a gunshot as composed of the initial muzzle blast which is followed by the sonic boom of the [supersonic] projectile. The perception of the sonic boom is limited to the cone of influence (aka cone of silence, mach cone), which is defined by the local speed of sound and the projectile mach number.
The muzzle blast can be thought of as dropping a stone in a pond. The resulting ripples will propagate outward and reflect off of any surface they encounter. If you have a plain muzzle, this acts as essentially a point source. If you have some type of flash suppressor or brake, the expansion happens at the cutouts in the device as well as at the end.
Well, the device as illustrated in that blog post is essentially like the first stage of a suppressor because it provides an initial expansion volume for the muzzle blast. If you extend that cavity forward a few inches and add some baffles --- Abra Cadabra, You just manufactured a suppressor!!
[knock, knock] Sir, This is agent ?%^& with the $%^"@!
The gap in the device illustrated is what makes it different than a flash suppressor or muzzle brake. The gap allows for some initial expansion, which is then directed downrange by the shield. It's basically the Krink and Linear Comp taken to the next level. I'd imagine that next level is a gray area between NFA Land and non-NFA Land, or we would be seeing more of these devices.
A muzzle brake has surfaces which direct the compressed gas (and it's expansion) rearward or laterally, before it's introduced to the atmosphere and allowed to expand.
I'm just an aerospace engineer, but I would be hesitant to publicize that particular device within earshot of any upwardly mobile federal agents...