It's fine to call someone out on their opinion. That is what we do here, but it is pointless to debate whether someone can be certain about their opinion. If they are certain, then they will have nowhere to hide if they are proved wrong. Me, I hate wishy washy opinions with all sorts of qualifiers and contingencies. Put it out there and take you lumps if you are wrong, but proving to the person that they can't be certain about their opinion is silly.