Technique “rights and wrongs” have been long argued here and elsewhere.
When I was a tutor, I did introduce certain left and right hand positions for new starters. This was generally to help efficiency of movement and to hopefully avoid muscular or skeletal issues later on. However, it didn’t over bother me if someone had “unique” and entrenched hand or finger positioning, it was the students choice after all. If they were developing their musical palette, achieving the goals that motivated them and were enjoying playing the instrument, then fab!
CS’s thumb has probably been in all sorts of positions, and good on it, I say. I wouldn’t have told him where to shove it.