You might actually both be saying the same thing here. Caveat being that I have no idea what the science says about cold/flu, but when talking about humidity when the air is colder it is drier. 90% humidity at 35F is not the same as 90% at 85F. As the air cools it is able to hold less moisture. So your observation that there’s an uptick during colder weather at a higher relative humidity could be the same as saying it’s more likely in drier air, because the air is drier when it’s cold, even when the relative humidity shows the same percentage.
You might actually both be saying the same thing here. Caveat being that I have no idea what the science says about cold/flu, but when talking about humidity when the air is colder it is drier. 90% humidity at 35F is not the same as 90% at 85F. As the air cools it is able to hold less moisture. So your observation that there’s an uptick during colder weather at a higher relative humidity could be the same as saying it’s more likely in drier air, because the air is drier when it’s cold, even when the relative humidity shows the same percentage.