I definitely think they were clueless.
I know a bit about wine - I enjoy wine, I've taken a few wine courses (ranging from $20 classes with 100 other people and only 4 tastes of wine, to $300 per person courses that did food pairings), I've done winery tours, I've done tasting menus at upscale restaurants and added the wine option, etc. And I had never heard of Opus One before this thread. I've tasted a few $200+ wines in my life, but never on my own dime, its simply so far out of my range its just not on my radar at all. I usually cap out at $75 for wine and even that is only very very occasional, usually wine for me is $50 or less in a restaurant and $20 or less for home. I would assume the majority of folks don't just casually have uber expensive wine brands memorized.
That said, if I were to drink a wine I didn't recognize while pet sitting, I'd just google it on my phone. I wouldn't really think anything of drinking a bottle of Yellow Tail, or even Coppola if the wine collection was in that general range, but anything I didn't recognize, yup, google.
In my home, I have said to people cat sitting "make yourself at home. Help yourself to the wine and liquor except the Johnny Blue and the La Veuve Clicquot" as those are the only two I really am not keen to lose." I don't bother with a post-it, I just know those are my two special ones.