That has been the rule since I was a child. Sure you could get away with a bit closer to the time, but 90mins for an international flight, no chance. Even for a domestic flight that is pushing it.

And in Manzanillo especially so. The front desk staff for check in, also attend everything airside and there are rules, timings and proceedures to follow that take time and they are within their rights to deny you access for not following the rules established in the conditions of your tickets that you agreed to when you purchased them.