I've come across this phrase: "１００歩譲って". I couldn't find an explanation in English as to what it means but it seems to be in use. I assume it's something like "let's suppose, for the sake of argument".
Your understanding is correct. It suggests that even if part of the opposing argument is assumed true (by "taking a hundred steps back") the end result won't change.
Even if I would also go there if going on the demonstration would mean the military legislation won't be passed, (in reality) it still can't be stopped in any case and most of the demonstrators should know this too.