Seems likely, although as the paper points out, there are a few other possible causes too, like use of pretty powerful insecticides on many routes.

The air in a pressurized aircraft cabin is often unhealthy also, due to contamination with lubricants/hydraulic fluids in the engine bleed air.

Please call this “fume events”, as the risk is desperately concentrated on a few flights in a dozen years, and fume events are not yet recognized as events (“just renew the air and no-one will see” - while we have amateur testimonials that people get conditions for life after such events).

Where can I read more about this?

the unorthodox sleep schedules required to fit flight schedules is also probably a contributing factor. night shifts and irregular schedules have been correlated with negative health outcomes for a long time.

Sure, if I had to guess something that wasn't radiation, I'd guess stress, which I would guess could depress your immune system and make you more susceptible to C19