Without seeing the video and having the context of his statement, it’s kind of hard to be sure but I’d say it is very unlikely that he is reusing PKM barrels (or any gas operated machine gun barrel) on a Mossin.
For one, the Mosin barrel is threaded into the receiver while the PKM (and most other belt fed machine guns) are using some method of quick change mechanism. That means he would need to put the barrel on a lathe and reprofile and thread the barrel so that it can even attach to the Mosin receiver (assuming there’s enough material there to begin with) and he would almost definitely have to touch up the chamber with a reamer to ensure his headspace is correct.
Additionally, he would also need to weld up the gas port, mill out the area for the rear sight to attach, as well as add a new front sight.
All of that is going to require a lathe and mill, as well as almost the exact same steps (indicating the bore, chambering, etc) as starting from scratch, so he might as well just start with a barrel blank. There’s no reason to have to do MORE work in order to just have a heavily used, lower quality barrel.
If I was going through the trouble of rebarreling a Mosin, I’d honestly start with a standard .30 cal barrel blank (so grooves at .308 rather than .310), chamber to 7.62x54r, and just use handloads with .308 bullets.
