Striderman,
I heard the same recommendation from the T/C service department after I sent them a 209x50 pistol barrel with corroded threads in the breech end of the barrel. I had purchased it used for a low price (I discovered why!). The previous owner may have used bore butter to lube the threads for storage or applied absolutely no protection after a not so thorough cleaning. The T/C gunsmith said that T/C would not like to have him say this but, he did not recommend bore butter and added that I should use Hoppe's oil and to keep the threads lubricated.
I now use #13 and bore butter when actively using my muzzleloaders. When I store them I clean them out with hot soapy water, dry, and apply oil to the barrel and lubricate the threads with thread lube. I also leave the breech plug off or the nipple off to allow air to move through the barrel. When I use a muzzleloader after removing it from storage, I clean the barrel with hot soapy water, dry it, and apply bore butter.
I would experiment with bore butter on your sabots. Some sabots may not care for any bore butter.
If you read the label on #13 or bore butter it will state it is "food grade rated" or "FDA food grade rated". In other words, you can eat the stuff. I have not verified this but I was told that McDonald's uses the same stuff to clean their grills. I now use it to clean our coated pots and pans and it does a great job. Something about storing food in my barrels that does not seem O.K.