I think the chance is a lot lower than you think, for two reasons.
Firstly, the podiums that each spoke at were at least 4-5 meters apart, far more than the 6 feet (2m) suggestion.
And secondly, it takes time to become infectious, several days. The likelihood is that he's not even infectious yet. He had to take a test prior to the debate, so his exposure was between that test and this new one meaning that he was very unlikely to have been in a position to spread the virus on the stage, even if he had actually be exposed to it and had it in his system at the time.