Hops definitely can add bitterness below 170F. While the isomeraization of alpha acids at lower temps doesn't seem to have been much studied (but see
An Analysis of Sub-Boiling Hop Utilization on the AlchemyOverlord site for an interesting read),
bitterness also comes from oxidized (non-isomerized) alpha/beta acids and even polyphenols.
Mark Malowicki studied temps of 90C and above.
https://ir.library.oregonstate.edu/downloads/bz60d000z And although he didn't look at temps below 90C, the curves extrapolated from his work seem to work nicely (subjectively and as supported by the analysis by AlchemyOverlord). I use a formula derived from Malowicki's work in BrewCipher's post-boil hop addition calcs. (To be clear, I use a formula I derived from Malowicki's data, not AlchemyOverlord's formula, but they yield similar results. See Figure 4 in the AlchemyOverlord writeup at
An Analysis of Sub-Boiling Hop Utilization for the money shot.)