Reasons for choosing a transistor instead of an op-amp as a buffer
From a pure engineering point of view an opamp circuit can be made such that it will have less distortion than a simple emitter follower as shown in your schematic. I can build both (in real life and/or in a circuit simulator) and prove that with hard numbers.

That is not to say that the simple emitter follower is worse or better (for audio) than an opamp based solution. What I can measure/simulate are only certain aspects.

Some prefer Tube amplifiers which most of us know add distortions. So the most distortion free solution from an engineering point of view might not be the best sounding one.

Regarding your simple emitter follower: as commented, it is a well known, simple to use, robust, proven, low cost solution. There is no reason to use an opamp. The lower output impedance isn't always needed. And low enough is low enough. There is no need to go lower than "low enough" now is there?

I am quite skeptical about this

and you should be, the emitter follower solution does the job, no need to make things more complex than needed.