In my cosmology lecture course, we derived the temperature of the cosmic neutrino background as
\begin{equation} T_{\nu} = \left(\frac{4}{11}\right) ^ {1/3} T_{\gamma} \,. \end{equation} Since the temperature of the cosmic microwave background today is $T_{\gamma}=2.73K$, the temperature of the cosmic neutrino background today is $T_{\nu} = 1.95K$ according to the above equation. However, my lecturer mentioned that we believe that neutrinos have recently become non-relativistic. But doesn't the above equation assume that both temperatures are inversely proportional to the scale factor, which is true only for ultra-relativistic particles? So once neutrinos become non-relativistic, $T_{\nu}(a)$ changes and the above equation is not right and we cannot say that $T_{\nu}=1.95K$ today. What am I missing?