I don't think what they're doing violates the law[0], but they could easily subpoena Reddit and find out who the users are if they wanted to.

[0]: too complicated to get into here, but basically my interpretation of the law is that you need to be making trades that you intend to cancel or revert immediately in order for it to apply to you, and what these traders are doing is instead speculating on the price going up and hoping to cause the shorts to liquidate their positions, thus further driving the price up. Not spoofing to create fake demand.

How would Reddit know who those people are? Protonmail + VPN would hide their identities as far as Reddit is concerned, right?

In theory. I'm not sure if Protonmail or the VPN provider would respond to a subpoena, but I wouldn't rule it out. But realistically most of these people are using their home IP address and their gmail account.

There are VPNs not bound by US law and the same with email providers (if your email address with Reddit is even legitimate to begin with).

Yes, but it can't be more than a few percent of people who go through the effort to set that up right? I know it's not hard to do (I've done it), but I think most people on reddit are using either their personal email or a throwaway gmail account they use from their home IP.

You actually still don't need an email address to create a Reddit account.

Reddit has stayed true to their word of maintaining the old design, and it's free of the dark patterns in the redesign. Go to https://old.reddit.com/register and just leave the email field blank or if you use the pop-up menu (the thing you get when you click login/register in the top right of the old design), you can click skip on the first screen where it just asks you for your email.

I would not assume that de-pseudonymizing Reddit accounts is impossible, especially if large amounts of money are on the line.

Reddit complies with FBI requests to hand over account data.

Indeed. In many cases, you can get far just by looking at post and comment history. You can tell what town they live in and see screenshots of their texts and FB posts (albeit with names and faces blurred).