I never heard that one. Makes sense to me.
That is the problem with non-believers, they cannot imagine the World and its atmosphere different than it is now. They will say something like, we only live to about 100 years old at the most, but the Bible says people lived 900 years, therefore it is fake.
It did not rain for some 1600 years, the Flood was the first time it rained. Dew and/or water coming out of the ground kept vegetation growing.
The high oxygen content was very healing. I believe the atmospheric pressure was much higher too, maybe twice what it is now. The combination of higher oxygen content and higher atmospheric pressure is the basis for wound healing using a hyperbaric chamber.
The original creation was designed to provide an environment for Man to live forever. Sin changed that, but up to the time of the flood we had an environment conducive to long life.