Giant prehistoric insects could only exist because the air held more oxygen

What actually happened

The old idea rested on the assumption that oxygen delivery was the bottleneck capping insect size. The researchers tested that directly by measuring how much room the tracheoles, the finest air-delivery tubes, take up inside insect flight muscle across a huge range of body sizes.

The result undercut the premise. The space devoted to those tubes rose only about 1.8-fold across a 10,000-fold span of body mass and stayed around 1% of muscle volume or less, including when the analysis was extended to Meganeuropsis, the giant Permian griffinfly. In other words the plumbing had plenty of spare capacity; the animals were not straining against an oxygen ceiling, so a richer atmosphere was not the thing that let them get big.

This is disproved in the specific sense that matters: the confident textbook causal claim (high oxygen is why the giants existed) no longer stands. Something else, perhaps the later rise of agile flying predators like birds or physical limits of the insect body plan, must explain both their size and their disappearance. The griffinflies were real; the tidy oxygen story behind them was not.