I have asked this question myself before, especially when I have been around people building custom motorcycle exhausts.
Some of the better performing pipes have a hole straight through the center that does not restrict the air flow, baffles around, and really are essentially a suppressor already.
So in that case not only are all the parts initially present, but they are then assembled into what is essentially a suppressor, except with the intent of attaching it to exhaust pipe and not a firearm.
The ATF definition of something that reduces sound by 1 decibel or more is also meaningless and arbitrary. A few inches of firearm barrel will reduce a firearm report by more than that, and in some cartridges a single inch will.
For example 12 gauge:
12 Gauge 28" barrel 151.50dB
26" barrel 156.10dB
18 _" barrel 161.50dB
Just the two inches between 26" and 28" length barrels are a change of almost 5 decibels!
So by that definition anything that extends the length of the barrel but is not the barrel would be a suppressor, like those fake silencers that are hollow tubes legally sold for asthetics all the time. Yet the don't declare those are suppressors. So it really is entirely arbitrary. If they want to declare it illegal just about anything will reduce the sound by 1 decibel or more.
Oh and just for correction, even though I referred to it as a suppressor and use it interchangeably with silencer as well, the legal definition is 'silencer'. Which is also what the inventer of the device (who also invented the automobile muffler and similar industrial devices) called it before legislation cemented the term in federal law.
We can refer to them as suppressors, but when discussing the specifics of the law what the law actually calls it and is defining becomes important.