In my view, from having studied and worked extensively with evidence-based relationship and marital counseling, this is more of a folk-explanation then a truth.
Each human being is so complex and unique that there are bound to be countless apparently opposite and similar features that can be associated with what is attractive vs opposite.
I think in most cases, saying that partners are opposites signifies that they lack core relationship maintenance skills.
If more couples new how to effectively express, hear and meet each others' basic relationship needs, then they would not view themselves as opposites. Nor would others.