That has everything to do with Red Auerbach not the city of Boston. I think it is pointless for someone to argue whether or not one city (Philly) is more or less racist than the other (Boston). But it is ridiculous to use Red Auerbach being a good and progressive man as evidence for Boston not being/used to not be racist. People literally broke into Bill Russell’s (the first black coach you alluded to) house vandalized his belongings and defecated on his bed. There is a reason Russell didn’t stick around after his retirement.

Russell prevailed in the face of aggressive and relentless racism, particularly during his playing and coaching career. Even as the Celtics were winning championships with him as their centerpiece, racist locals in Boston routinely vandalized his home.

And there are many reports from players about Boston fans using racial slurs towards them (LeBron, Draymond, Marcus Smart who plays for the Celtics). Now, the acts of the few do not characterize an entire city, and the common discourse among NBA fans about “your city is more racist than mine” is asinine, but when all of your reasons for Boston not being racist is because of things Red Auerbach did then you don’t have a strong argument.