I saw it in the news when he spoke about that. The doctors who were sitting on the side didn't know how to react when he questioned them about it and T said it has not been done yet but might maybe be interesting to do some research on it. He also spoke about light that kills the virus.
Here is the proof:
Go to 00:36.
He wasn't joking.
You have to understand...I watched every briefing he gave, live, since this shit started. Why not? I work from home and have time on my hands. You can tell when Trump is joking, because people in the audience actually laugh. Nobody laughed there.