If he wanted to fire anyone who disagreed with him, this EO wouldn’t have been necessary. With very few exceptions, executive branch employees serve at the pleasure of the President.

This is how it is, how it has been, and is entirely consistent with the Constitution.

So, if not that, then why issue this EO?

First of all, it’s a statement: “Resistance to this agenda from within the executive branch will not be effective”

Secondly, it helps ensure that when the President issues a statement, it’s not immediately met with bureaucrats making statements to the contrary.