This is not saying the court decided anything - it is just saying it is allowing commentary from mods to be submitted to the court, without knowing their identity.
How, exactly, might things "go wrong" for folks contributing to an amicus curiae brief[0]?
While amicus briefs are common, they're just arguments for or against a particular issue in a particular case. They aren't testimony, they aren't sworn affadavits and they have no legal weight.
It's essentially writing an OpEd or posting a blog post, except it's filed with the court. No one is "testifying" (in fact, appeals don't have witnesses or evidence or anything of the sort, just a review of the proceedings of the trial court, plus any arguments the lawyers want to make) in an amicus brief and no one is held to any particular standard.
And so I ask again, what, exactly, could "go wrong" here?
This, Reddit’s spokesperson notes, is “a significant departure from normal Supreme Court procedure.” The Electronic Frontier Foundation, a nonprofit defending online privacy, championed the court’s decision to allow moderators to contribute comments anonymously.
“We’re happy the Supreme Court recognized the First Amendment rights of Reddit moderators to speak to the court about their concerns,” EFF’s senior staff attorney, Sophia Cope, told Ars. “It is quite understandable why those individuals may be hesitant to identify themselves should they be subject to liability in the future for moderating others’ speech on Reddit.”
I'd note both the EFF and Reddit said this was unusual.
But then we live in a society that birthed Twitter.