You are technically right, but from my experience the size community really doesn't see the difference. Sure, from the perspective of what's around the main female, she is not a giantess, but from the male's, she is a giantess.
On a logical level, there isn't much of a difference between the tags and their results, however one suggests a minority of giants and the other suggests a minority of tinies. In the end, it's an arbitrary assessment, though. Who's to say they aren't giants in a giant room? There isn't a line that clearly defines whether something belongs to one or the other. It isn't as if these tags were created with logic in mind, they just naturally evolved. Granted, I don't care either way. I will take both, but this is a case where miniguy is quite certainly the appropriate tag.