This was meant to be about "good" rather than olive oil, but if we accept that (in the specific case of olive oil) "good" = "extra virgin" (and I have yet to see any confirmation of that), I suspect that Nigella never shops in Asda.
TV chefs do it all the time: "use a good xxxx" - without any guidance on what constitutes a good xxxx. If the viewer knew what a good xxxx is, they wouldn't need to be told to use a good one. The relevance is that if the best quality possible costs ten times more than a lower quality that is nonetheless adequate, one needs to know where the threshold is if one is on a budget. If the chef means "the best you can afford", even that has difficulties: what if the best you can afford is actually crap and will ruin the dish, but because you have never bought the expensive stuff you don't know that?
It's like that with me and champagne: do I not like it because I don't like champagne, or do I not like the champagne I am prepared to pay for? Do the people who say they like champagne actually not like it, but say they do because you don't spend £20 a glass on champers (or accept a £20 glass from someone else) and then say you don't like it?
Another case is when the chef says "xxxx is best, but if you can't get xxxx, yyyy will do just as well" - no it won't! If xxxx is best, then yyyy cannot possibly do just as well. It might do nearly as well, but not just as well.
I tried to bring up a new subject a few posts ago, but nobody noticed.