Answer
Does one publisher apply the same waiver policy across all its journals?
Not always. Sage notes that certain journals use World Bank income levels rather than Research4Life. IOP notes that journals published on behalf of partner organisations may differ. Springer Nature's country-tiered pricing pilot supersedes waivers on participating journals but not on the rest. Policy therefore has to be checked at journal level, not only at publisher level.
Detail
One trap worth knowing
Sage states that opting into a technical edit on certain journals makes an article ineligible for a waiver — a choice made during submission that quietly removes the relief.