I'm not sure where this "it's a short barrel so it has to be cut" comes from.
Shotgun barrels are made from blanks, just like rifle barrels. A barrel is not considered a barrel until it has been FINISH REAMED and any extension installed. (that's how you measure it, can't measure if you can't mate up a bolt face)
So that short barrel gets cut off from a big long shotgun barrel blank, then turned on a lathe to profile. My god would there be a lot of waste if they just took 28" barrels or whatever and cut them down

In fact a shorty barrel should be cheaper because you can make more with a single blank
Dlask likely just cuts shotgun barrels the same way, cut off chunk from a blank, turn down and machine, then ream chamber and forcing cone, then finish ream and install barrel extension.
And yes, if a barrel comes in from the USA and it's been admittedly cut down, it's prohibited. The caveat to that, is how do they KNOW it's been cut down? Very much impossible to say so most people - including the RCMP - turn a blind eye to it, because it's nearly impossible to tell a professional job from a newly manufactured one.
I'd bet there are hundreds or even thousands of barrels that are cut down past 18" and technically illegal (including those from before that was a law) that are technically illegal, but impossible to tell.