If there is no power or risk on the part of the employer then why do some countries have a law requiring that you give the employer a notice period? It's to reduce their risk.
In my country can sued for their damages if you leave without a notice period - real damages such as having to hire more expensive temporary staff to do your job until they can find someone else permanent. That's their risk.
In the linked article, it's mentioned that Elon felt a constant need to fire people that he sometimes had to be talked down from, so if the employer risk exists, it doesn't seem to be particularly high in most cases. If the risk for employers was significant, then at-will employment laws wouldn't even exist, because the entire purpose of such laws is to make it arbitrarily easy to fire employees.
The only way at-will employment could truly be symmetrical is if I could walk into work and literally fire any of my managers or the CEO with the same ease with which they could fire me.
But of course, actually applying the power dynamic of at-will employment both ways would just be insane wouldn't it?
That's not symmetrical, or if it is, it's symmetrical in the same way as "heads I win, tails you lose." Being fired and quitting both place the entirety of the risk of unemployment on the employee, there is no counterbalance of power or risk on the part of the employer.
If health insurance wasn't tied to employment in the US, or if there were something like UBI, then the dynamics of at-will employment might be at least more symmetrical.