Its just a theory that the mouth spreads the virus. Theoreticians have been all over the board on this, that certainly wasn't the theory 4 months ago.
Yeah... Just like Gravity is a theory too.
Coronavirus Disease 2019 (COVID-19)
The virus that causes COVID-19 is thought to spread mainly from person to person, mainly through respiratory droplets produced when an infected person coughs or sneezes. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Spread is more likely when people are in close contact with one another (within about 6 feet).
That's the CURRENT theory. Past theories (from not that terribly long ago) suggested that it was picked up off of surfaces (thus the need to wash your hands after using the can) Considering how much it goes through SOME nursing homes, its certainly possible that catalysts to spread the virus are in SOME plumbing systems.
Pretending that the libs at the CDC have all the answers is sort of stupid, even if these fellow are really insistent on it.