Plagiarism generally refers to someone intentionally stealing someone else's work and not crediting it. However just because two pieces sound similar unintentionally doesn't mean that it's necessarily right either.
Actually here I do think that they'd have grounds to accuse too much similarity. Not only is the progression the same, the phrasing is damn well near the same too. If you turn in a plagiarized paper with a couple words and sentences changed it's still plagiarized.
Of course, here we're talking about a shitty C-ute album track and a shitty Kpop single.

That melody is so old-fashioned(in a bad way) it isn't worth fighting over.

Buy hey, I wouldn't mind C-ute getting free publicity from this.