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I'm under the impression that a higher prehistorical concentration of oxygen in the atmosphere is widely accepted at this point. Higher atmospheric pressure is not something I've heard anything about though.

Wikipedia has a nice graph showing high and low estimates for the historical oxygen level: https://en.wikipedia.org/wiki/Geological_history_of_oxygen#/....

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I don't think anybody is claiming that Earth gained extra nitrogen (how would it?). So, while it lost oxygen, it also had to lose some air pressure.

I imagine there was some huge pressure loss at the oxygen catastrophe.

My thought would be, when the asteroid hit, it blew away a large chunk of atmosphere with it.

You also need to account for CO2.

There is almost no CO2 on the atmosphere.

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