No, the premises
always need to be examined, not accepted at face value.
If the author is using a wrong definition as a starting point, the rest of the argument is largely pointless...or must be explicitly marked as a hypothetical/counterfactual.
"I define the earth as being an infinite flat plane, from this it follows [for the real world] that..."
Well, no, the earth is not an infinite plane, even if it kinda looks that way, was long believed to be that (maybe minus the infinite) and apparently there still are people who believe that.