That certainly may be one of the explanations of the reduced severity we're seeing but to the OP's point, wearing masks, social distancing, etc. should also result in fewer cases yet the "official" numbers (at least in NJ) say otherwise.

I still strongly suspect that NJ's high water mark from March/April was a fraction of the real numbers back then and today's case counts are closer to reality due to testing availability. The same goes for NY. I'm sure SOC has improved too but that can't explain all of the differences.