We only have two electoral systems. If we want to go down this route, I think it’s easier to codify what we want in each case (voting approval in an IRV election, voting IRV in an approval election) than to defer to common sense. Frankly, we’ve gotten into sillier arguments before, and spelling out something like “anything ranked above RON is considered approved” or “anything given a ranking is considered approved” is not that many words.
I’m not actually sure how we’d interpret an approval ballot as an IRV ballot though. I don’t think a reasonable person can clearly infer ranked preferences from unranked approvals? Maybe the current assumption is that this is not the general case, but it’s exactly what happened last time.