Undetectable = Untransmittable has been a mantra in the field for years because there is good data that people who have 0 detectable circulating copies of the virus cannot transmit it.
Transmission rates: https://www.cdc.gov/hiv/risk/estimates/preventionstrategies....
The second link shows transmission rates of 0.00 (zero transmission) in every study of many studies for undetectable.