[semimojo]

Now if a law has been passed ahead of time that defines the powers of the executive officer (president or governor) then he can issue orders within those bounds.

That's right, and I'm no expert in Texas law but Abbot chaims that's exactly what's going on here. From his original EO:

"WHEREAS, under the Texas Disaster Act of 1975, “[t]he governor is responsible for meeting . . . the dangers to the state and people presented by disasters” (Section 418.001 of the Texas Government Code), and the legislature has given the governor broad authority to fulfill that responsibility."

[Magnum44]

... broad authority ...

Thats a very dangerous wording. But I've seen it used elsewhere dangerously as well. Courts in child custody have "broad authority" or "great discretion" which has been often abused.