No, the definition of a monopoly is "the exclusive possession or control of the supply of or trade in a commodity or service."

A company that is not easy to undercut could just be an efficient business that operates roughly at cost without excessive margins.

I disagree. Spotify is practically a monopoly and is a rent-seeking entity. The reason artists can't go somewhere else is not because Spotify efficiently operates at cost (their gross margin was 26.5% in Q4), but because it's the only brand most customers know.

Deezer is in competition, Amazon Music, YT Music.

Monopoly doesn't apply, does it?

Even DAB/FM radio takes their lunch.