He actually never joined the movement. Hhe served as chancellor at its seminary. 2 different things. There are Orthodox scholars who teach in universities and non-Orthodox institutions. If anything, they have a good hashpa’ah on those who are not yet frum.
Regardless, my hope is that R’ Stav is elected Chief Rabbi, and I think that ultimately, the humbling that the Haredi parties will experience will do them good in the long run.