Here are the actual guidelines: https://www.cdc.gov/coronavirus/2019-ncov/community/ventilat...

Note that when they say "5 ACH" they include both actual air changes and "effective air changes" due to filtration or germicidal UV:

"Some air cleaning and air treatment devices do not bring in outdoor air. Instead, they clean or treat the indoor air to reduce the concentration of infectious particles. Thus, they give eACH without the need or expense of conditioning outdoor air."