None of us are genre authorities anyway so it's just debating opinions. I don't think any of us are right in any objective sense but that's how I make sense of it. I know I've probably got popular opinion against me. You're right that I don't consider Univers Zero to be prog rock. At least not what I've heard of them which was an orchestra playing avantgarde chamber music.
edit :
By the way, "the line between prog and avant-garde is stupidly blurred" was very well put
edit 2 :
I feel there should be a distinction so that prog does not simply mean avantgarde because then the whole "prog" expression is useless. You might as well say avantgarde music then.