Here's the amicus brief: https://www.supremecourt.gov/DocketPDF/22/22-293/242292/2022...
Later on in the brief they get all self-referential and point out how this brief itself exemplifies parody. But this isn't a parody of a legal brief. It's a real legal brief with arguments and citations that might really sway the court, with some parodic elements.
I'm not. Even if one is entirely unfamiliar with The Onion, the final paragraph of the first section makes it pretty clear what they're about and why the document they're submitting is written the way that it is:
> The Onion files this brief to protect its continued ability to create fiction that may ultimately merge into reality. As the globe’s premier parodists, The Onion’s writers also have a self-serving interest in preventing political authorities from imprisoning humorists. This brief is submitted in the interest of at least mitigating their future punishment.
And, as the titles(?) of Arguments I and III note
> Parody Functions By Tricking People Into Thinking That It Is Real.
> A Reasonable Reader Does Not Need A Disclaimer To Know That Parody Is Parody.
You're correct, yes.
> Unless I'm the gullible, unreasonable person who was tricked and still doesn't fully get it.
You're -I presume- neither more gullible nor more unreasonable than typical for humans, but no, you still don't get it.
A huge part of the practice of law is persuasion. What better way to persuade someone that the laws and regulations that have protected parody for ages are good, prosocial, and should remain intact than to gently use parody as part of one's document of persuasion?
If your position is that parody is strictly prohibited in a legal document, then what's next on the chopping block? Metaphor? Simile? Colloquialisms?
I was surprised that they didn't contribute to the jokes, it really does read like it was written by a particularly funny lawyer.
Who knows, if they've been around for 270 years, they might use troff like research papers.