Zero used metal drop tanks during the early phase and then had to switch to wooden drop tanks later since aluminium become too valuable. Also, I’m fairly sure that early successes of Zero fighters didn’t influence the discussion about the fighter range. At that point, it was assumed that Japanese aircraft carriers had launched attacks by sailing close to targets at both Pearl Harbor and Phillippines and many didn’t realize that the attack on Phillippines had been from Taiwan. It didn’t help that both attacks had been using the same type of Zero with the only difference being Phillippines had been bombed with clearly land based twin engine bombers.

One reason that US generals had been overconfident may be that only US and UK fielded a large number of 4-engine bombers while almost all bombers of Axis were 2-engine bombers. (Italians had 3-engine bombers but they weren’t any better.) Since 4-engine bombers could and did carry more armaments, they could have assumed that 4-engine bombers would have a higher survival rate while inflicting more damage to intercepting fighters.