Apparently the fools on the pro-gun side argued only a point of minutia: that the P80 kit didn't fall under the definition of "readily convertible" to a firearm based on where the definition fell in the text. The ruling was narrow in that it held some kits meet the definition ("we'll know it when we see it"). They didn't even challenge the constitutionality of the law under Bruen's text/history/tradition test.

When you challenge a law, you list every possible reason you claim it should be struck down, because you never know which argument will be the winner, and you're stuck with the arguments from the original trial on appeal.

The Obamacare lawsuit was the model for how you do it. The Republicans threw every objection they could think of in the filing, which was a good thing, because they won on some obscure Medicare cost sharing rule instead of the primary argument that it was illegal for the feds to force you to buy insurance from a private company.

The court ate up the bullshit claim that the penalty was a tax, even though it was always called a penalty until they were defending it in court. It would have been game over right there if that was the only argument in the filing.