Wonder if he originally played slide, learned to solo with the slide still on, then just never got away from that technique?
I play with the slide on my ring finger, and am not about to change.
I usually have to shrug a folks who have bold "you're not doing it right" statements about guitar and setups. While these usually revolve around ignorance behind using a capo, anything from string gauge, tuning, pickup wiring and config, to tone woods, branding, fret radius, etc. also gets commented on like there are laws.
It's (usually) wood and strings. It makes a noise. While certain combinations historically make the noise more pleasant, and tendonitis is a thing, no one has the authority to deny the artistry of one noise over another.