FIRE is definitely about income just as much as it is about being frugal and saving. Having a high income is what enables the RE part.

There is a base level beyond which you can't save much, so first order of business is maximizing your income (e.g. better job/raise/promotion) without going bananas and sacrificing your health for it.

That would not work well in the US with annual out of pocket healthcare expenses that can be up to $21.2k per year per family, or $10.6k per year for a single person.

Plus the monthly insurance premiums. Financial independence without a large sum of money does not make any sense, and a large sum of money comes from either inheritance, or income.

Obviously you need money and obviously you get it from income. But it is easier to reduce your expenses than to increase your income, and reduced expenses also result in excess income even with no income changes.

Yes there is a floor to this strategy. If you are going to the food bank to feed yourself because you don't have enough income you're unlikely to be able to reduce expenses enough to make this happen. But if you're lower-middle-class or above it is possible.

It's about controlling expenses after having a large income.

With a small income, everything goes on necessities. FIRE isn't possible.