> That graph does not describe codec efficiency.

My friend, the vertical axis is literally labeled "Quality", and the horizontal axis "Bitrate". The caption is "The figure below illustrates the quality of various codecs as a function of the bitrate." Quality at a range of bitrates is how codec efficiency is measured.

I'd never heard the claim that "Opus is basically transparent at 128 kbps", but I did find https://wiki.hydrogenaud.io/index.php?title=Opus, which agrees with you: "Very close to transparency". But it also notes, "Most modern codecs competitive (AAC-LC, Vorbis, MP3)", which lines up with the chart.

Early Opus vs. MP3 tests were done with LAME, which is awful. This may be why you're under the impression that MP3 needs twice as many bits to get the same quality.