The reference to “all other atmospheric gases” only applies to what’s going on in C-4 plants. It has nothing to do with non-C-4 plants. In fact, the author never does explain why “common atmospheric conditions” can cause oxygen to build up and hinder photosynthesis in non-C-4 plants.

This claim is never made in the passage. The only gases discussed are oxygen, hydrogen, and carbon dioxide—and of the three, only oxygen is identified as interfering with photosynthesis. Hydrogen and carbon dioxide, meanwhile, are both beneficial for photosynthesis.

The author doesn’t reach this conclusion. She never suggests that C-4 and non-C-4 plants differ in which gases they use. Rather, she indicates that both types of plant make use of the same gases: hydrogen and carbon dioxide. Her conclusion, meanwhile, is that C-4 plants are different because of their ability to shut out oxygen gas, which neither C-4 nor non-C-4 plants make use of.

It first raises a problem: if “all other atmospheric gases are excluded,” then carbon dioxide can’t reach rubisco as a gas. The existence of this problem, in turn, explains why C-4 plants have had to develop a solution: carbon dioxide undergoes a series of transformations in order to covert it to a non-gaseous form, allowing it to reach rubisco and participate in photosynthesis.

The author doesn’t make this claim. She never suggests what the oxygen levels might look like in C-4 plants. Rather, she just indicates that oxygen is present to some extent (due to the splitting of water) and then focuses on how oxygen is kept physically separated from rubisco.