A kit muzzleloader long gun, like a kit model plane, is an assembly, not a build. This is not a bad thing, it is what it is. Not everyone has the skills, and/or tools, and/or time for such a venture. I'll add - there appear to be "levels" of kits, some are literally screwdriver built, others require varied levels of skill, tools and time.
To build (or "make") a long gun depends on one's definition of those words, particularly the word "make". In terms of a long gun build, will all the components be created from sourced raw materials? Where does one draw the line at "raw materials"? Will it still be a "made muzzleloader" if one purchases one or more pre-made: barrel, lock, double set trigger, castings for butt plate and trigger guard and nose? If one has a plank of wood for the gun stock, will the completed stock be an equally "made" stock no matter if a CNC machine was used or manual chisels, rasps, and drill? Are there levels or categories of "made" guns?
What is considered to "make a muzzleloader"?