I will note that chokes without parallels are nothing new. Lefever as well as several other makers were using "Taper choke" which meant the entire length of the choke was a cone with no parallel section, some of them beginning well back into the 1800's.
Adjustable reamers have been offered to the trade for many years with the instructions to use from the muzzle & take very light cuts at a pass & let them follow the parallel.
On the taper choke there is no parallel to follow. I worked as a machinist for over 35 years & will assure you. I don't care how good you "Think You Are" you cannot align an unguided reamer with the bore.
As rocketman said it truly matters not whether the reaming is done from the breech or muzzle as long as the reamer is aligned with the bore. It also matters not whether the reamer is hand or power driven. In the linked video the reamer was inserted from the breech with a bore fitting bushing on the shaft. In this case the reamer itself will center in the cone with the bushing keeping it in line. The fact that the reamer was turned with a heavy tap wrench is of no consequence.
To ream from the muzzle does require a more elaborate set-up as both the barrel & reamer, for a satisfactory job, must be held in alignment.
