Driveability is what counts.

Guess he decided to run a bit leaner for eng reliability. Yes, a rich mix apart from causing emission issues, can make eng run hotter and be to the detriment of engine longer life.

Have read that after market turbo diesel run best around 19-20 AFR or higher under full load and a factory turbocharged diesel of about 20-22:1 or higher under load. Not to say that you cannot run a richer mix many do.

My CRD procharge stage 3 h/f turbo, runs Max boost 26psi and 19:1 AFR. Came down to 19:1 AFR from 21:1 for more fuel low down to achieve improved early boost (getting there now the need to further improve induction).