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.

TLDR: This case isn't settled this change was just a matter of if a party should have been subject to an injunction before a ruling is issued. This isn't major news, the SC isn't commenting saying it's cool or not cool to regulate X. This is them opining on a procedural mater.

Also a layperson, but while the courts may have ruled against certain details of background check requirements (like whether certain kinds of restraining orders can be used to disqualify someone from buying or possessing guns) they have not ruled against background checks being required for gun sales as a concept. The entire purpose of these kits, whether the manufacturer says so or not, is to bypass background check laws by selling something that technically doesn't meet the definition of a firearm but can very easily be modified to become one.