They can deny you boarding if they merely suspect you have coronavirus, or if they want you to obtain a test showing you do not have the virus and you decline. What will lead them to suspect someone may have the ‘rona? It could be temperature checks. It could be data shared via an app or the government in the future. It could be information about known exposures, including on a prior flight.
Consider this part of the new normal that we’re figuring out – we do not know what is going to happen in practice but United is updating its rules to allow them to take action based on what sort of testing or screening occurs.