Animals, when hibernating, enter state of torpor which essentially significantly reduces energy consumption. While humans normally don't have this, with this virus, it might be a good reasoning for the story to allow those people to still survive for weeks with minimal daily energy consumption which will primarily be obtained from fat. But this is really the best answer I have if you want to somehow make it somewhat scientifically realistic. Considering what the author did several chapters ago, I doubt he was thinking about this.
If you say to not trust the firefighter because you support the MC's decision back several chapters ago, then I guess sure. But in my opinion, what the MC did back then was, not only morally wrong, but also very risky and wouldn't have succeeded had it not for them being in a 'shonen manga'.