How is Air Quality Index relevant to both PM2.5 and NO2
Most locations in the US don't see high concentrations of pollutants like NO2 and CO anymore.  They are largely no longer an air quality problem and air is "Good" in regards to those pollutants.  Especially during "lockdown", when traffic is light.  This is largely thanks to tailpipe emissions standards for vehicles and a much more fuel efficient fleet. On the other hand, PM2.5 has multiple sources that cause high concentrations, and we are much more likely to see the AQI vary beyond "Good" PM2.5 (e.g. outdoor fires, wood-stoves, secondary aerosol formation).  

The AQI is based on health-based evidence, with a different concentration threshold for each pollutant.  These are all put onto the AQI scale for simple communication to the public. If concentrations of NO2 had reached levels of the NAAQS, then you would see your graph plot into those AQI levels (note NO2 doesn't go as far up in "AQI" values as PM2.5 on your graph because those NO2 concentrations were never detected by the monitor).  

The National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO2) is a 1-hour NO2 standard of 100 parts per billion (ppb). See https://www3.epa.gov/airnow/no2.pdf and page 2 here: http://njaqinow.net/App_Files/2016/AQI%202016.pdf which has the break levels for each pollutant, shown below: