But the data shows that conditions in the past were unlike anything seen today.
Take this article from Science Daily:
"These elevated [oxygen] concentrations have been linked to gigantism in some animal groups, in particular insects, the dragonfly Meganeura monyi with a wingspan of over two feet epitomizing this." (italics in original)Insects have an open circulatory system. This system is fairly inefficient, and limits the body size in insects today. In other words insects today could never be so big (they wouldn't get enough oxygen). So, the presence of these insects indicates higher oxygen pressure (either more oxygen or more atmospheric pressure or both).
Current oxygen levels are ~20%, "levels around 30 to 35%, as have been proposed for the Late Paleozoic". High levels of oxygen would also cause large amounts of coal to form.
Things were very different, that much is agreed on. That's what the evidence shows.