There are tons of reasons balance is important in a single-player game! The strength or impact of mechanics in games affect how valuable or useful other parts of the game are. Take food in ONI, for example. Stick a new plant, 'Tomato' in there. Say it costs 10g of water and 10g of fertilizer to farm, produces many of its own seeds and feeds into a simple recipe that provides high morale. The players of your game are there to solve problems, and the problem of "how do I get food?" just changed to "how do I grow Tomatoes" in most cases. You still have the rest of your robust farming / food system in ONI... just, most players aren't going to engage with it, because game players will take what they see as the simplest path forward, and not always the most fun.
In Radiowave's case, they're pointing out that there -are- environmental hazards in the game, and a whole system around them (Med bays, cures, and the works!) that players just don't have to interact with, because atmo suits are a near-perfect solution that is readily available and you're going to want anyways. The existence of atmo suits changes the design space of ONI. That isn't a good thing or a bad thing, it just is a reality of the game. Talking about nerfing atmo suits is valid, as there's room to broaden the design space of Oni, if done carefully.