The length of the 2nd Amendment is insanely short and likely thought to be quite obvious to the authors. Ironically, it has likely been more debated than any other Amendment. There have even been court cases that focus on how the placement of commas impacts the meaning.

To your comment on "well regulated," the debate there has to do with how the phase has changed meaning over time; well regulated meant "well maintained" or "taken care of." A well regulated clock, for instance, would have its gears cleaned and oiled at regular intervals.

Even in the groups that still hold that interpretation debate on whether the phrase then mean well-drilled/disciplined or well-stocked with arms.

With regard to at-home kits, the general rule/understanding was you could build your own with your own tools and any materials that were only 80% or less manufactured/machined to being a completed firearm.

The debate kinda went like this: "Is a block of metal a gun?"
"Well, no…"
"So… How much work am I allowed to do to this block of metal before I get in trouble for selling it to somebody else?"
"Ionno… A lot, I guess? 80% sound good?"

So, people started selling 80% kits within the bounds of the law. They were blocks of material mostly milled with instructions, and sometimes tools, to finish the job.

The article doesn't explain why these kits in question are getting blocked. I'm suspecting too many things were sold at once as part of the kit, though. 80% kits normally don't have barrels, for instance.