From the article

>In a statement to the newspaper, Magnus said he was asked by Homeland Security Secretary Alejandro Mayorkas to step down or be fired. He said he wouldn’t step down and defended his record.

Political appointees are basically "at will" and serve at the pleasure of the President.

If you cooperate when asked for your resignation you'll sometimes be allowed to resign with honor and dignity but if you don't the results aren't going to be pretty.

It's pretty rare for a political appointee to refuse a request for their resignation, though it has happened. When it does it ends up being a lot worse than if they'd simply resigned as requested.