Studies aren't there to cater to your ignorance. Burnout is well defined, and generally quite well understood by even the general public, this is not news. You switched from "soft science is all subjective nonsense" to "I don't like how it's written". You don't have to like it.
I don't think burnout is well defined at all - at least not as used colloquially. As I understand it there's a specific medical/ mental health condition called burnout that is pretty horrendous. But in everyday usage it can mean everything from that medical definition to "I've had a rough week". I don't see any reason to assume which end of that scale a survey respondent might be thinking of when they answer.
Any serious professional or academic study contains a "Definition of Terms" section that explains how the researchers are defining key elements in their study.
