It is a misconception to say that the time dilation here rules out tired light hypotheses (whether Zwicky's or others). I have often pointed out that time dilation has been found wherever redshifts occur, whether in SR, GR or even the classical Doppler shift (the latter being used to predict time dilation in SNe in the first place). Can anyone think of a case of a redshift which has not been so connected? We can reasonably expect to find time dilation when the tired light mechanism is finally pinned down.