I assume would fire an employee that's underperforming.

You haven't actually refuted the point of this article. You can do one thing better than two. If you believe in your startup, then you'll do it better working on it fulltime. Else, if you believe in the startup you're working for, then just focus on that. You will probably get better outcomes on either front.

This isn't a legal argument, it's general advice. He also didn't say you can't do a full-time job and a startup on the side. He said it's hard to excel at your job when you're doing a startup on the side.