"Suppose that we hit the body with a tremendous, whether it's ultraviolet or just very powerful light," Trump said at the White House coronavirus press briefing, adding: "Supposing you brought the light inside the body, which you can do either through the skin or in some other way."
It was absolutely not considered a scientific impossibility that UV light would have an affect on the virus outside the body.
"A Cedars-Sinai research team is in the pre-clinical stages of developing a technology that harnesses intermittent ultraviolet (UV) A light for treating viruses and bacteria. The technology has not been tested or used on patients. Cedars-Sinai has filed for patents related to the technology and has signed a licensing agreement with Aytu BioScience with the aim of potentially enabling near-term use as a COVID-19 intervention for critically ill, intubated patients."
It was a suggestion to explore it (all possibilities).