Also, why 100ml? Do you need 150ml to make the explosive? Couldn't there be 2 terrorists with 100ml + 50ml? All these questions, so little answers...
Regarding quantity, hard to find information, I guess they don't want to have a terrorist handbook to making explosives online, but I would assume that 100ml would mean multiple times this amount would not be enough to make an explosive large enough bring down an airplane.
In general, considering the overall cost of the measures, I would think that there is a valid reason and that "it does not make sense at first glance so it's just a security theater" does not hold.
What’s your sense of the overall cost of the measures? It’s not clear to me if you’re saying that high or low costs help justify them.