Ya got me. Good catch, thanks. As temperature decreases, density goes up, gas viscosity goes down, Reynolds number (at the same bike velocity) goes up, and the Schlicting friction factor goes down. But this is a second order effect, small in relationship to the overriding increase in drag due to direct increase in density.

As someone stated above, it may be that clothing has a more direct effect. I know I feel slower in a full-leg bib than I do in shorts.