I suspect the kind of opinion the quote is talking about is as defined here; a belief or judgment that rests on grounds insufficient to produce complete certainty.

Adherents of rule #1, however, will never have grounds sufficient to produce complete certainty, in which case, "fire is hot" is an opinion by that definition.

However, a a pure counterexample rather than a mere logical knot:

could you please provide alternative examples of conventional opinions that were also held in the past?

Sure. "Murdering your brother out of jealousy is wrong." That's a fairly conventional opinion, no?