The harm is when people take the metric as literally "what percentage of people who want to be employed can find a job", which U-3 emphatically does not represent.
It doesn't directly measure it, true. But the parent poster was saying it has extremely high positive correlation to measures that do measure it, so it probably doesn't matter.
By that logic should we also further broaden "unemployment" to mean "100% - employment rate"? That's a pretty common misconception as well, maybe even more common than the one you listed.