(not a lawyer) Employment is at will, but there are protected classes. Just like you can not be fired due to your race / gender, disability is also protected - https://www.eeoc.gov/disability-discrimination

Which is why they never tell you why they fire you. They just fire you or tell you that your position was eliminated in a re-organization that only affected you.

Yes, I'm aware, but I personally am in favor of pressing the issue. Though I don't know the details here, in some cases of first diagnosis there will be hospital admission with 72 hour mandatory holding (with limited communication). Instead, consider if someone had been fired for not communicating after being incapacitated in a severe car crash or for the ensuing months of recovery. What would your advice be in that case? What would the companies response have been? How accommodating would they be in the case of the crash?

And yet there are still thousands and thousands of employment discrimination cases every year, even though people doing the discriminating aren't saying "fuck the ADA" while they do it.

This isn't exactly false, but isn't the whole truth either. Your statements apply in general, but they don't cover some of the more detailed scenarios.

Employers are required to reasonably accommodate people based on disabilities and medical conditions. What is reasonable is up to debate. It would also require that you can show they fired you for that issue, which can be tricky because they can always find some legitimate reason to cover their true motives.

One example of this could be an alcoholic. Maybe they showed up to work drunk. If they tell their employer that they are an alcoholic, the employer has to offer to get them help before they can fire them. If they refuse help or the issues continue, then you're allowed to fire them. Now if they never told the employer about their condition, the employer could just fire them because maybe it was just a bad decision to stay out partying all night, which is not a medical condition.

This isn't the whole truth either. The employer isn't responsible for getting them help. But adjusting their schedule so they can get treatment might be reasonable. They don't have to say what the condition is.

Most importantly: The employee must take the initiative to communicate these requirements to the employer before the firing event.

It appears the OP may have waited until the termination meeting with HR to communicate the medical condition. It's not clear that the OP actually requested any accommodations or leave. In fact, it's not clear that the company denied the OP anything or had any way of knowing the OP's situation leading up to the firing event, due to the OP's admitted lack of communication with the company.

They said they disclosed the condition 2 days before they were fired.

Actually 3 or probably 4

They don't have to offer you treatment, just 'help'. That could be adjusting the schedule like you said, or other things.

True, you don't need to say what the condition is.

Getting someone help and helping them mean different things usually.

Firing someone for no reason is legal in most states. Firing someone for the wrong reason is illegal.

And being able to cite a legal reason when you're fired immediately after they learn of a disability doesn't necessarily give them a get-out-of-jail card. But it would vary from jurisdiction and the employment status here sounds complicated, so I would absolutely see a lawyer about it.