There's more than a few outliers, to the point where they're not even outliers at all. post #7158872, post #6810481, post #5548116, post #5536506, post #4717283, post #4828066, and the afore-mentioned post #4875980 are all from just the first page of results - almost half of that page doesn't qualify for head_under_clothes.
That being said, I do believe that all head_under_clothes posts would count as under_another's_clothes; I'd be in support of an implication. However, I also think the tag should be renamed to head_under_another's_clothes. With the name being ambiguous and the lack of a wiki, any genius could come along and start mass adding head_under_clothes to stuff like post #5354203.