Strictly speaking only if the first or last ticketed point is a US point does that DOT regulation apply.

But in practice it's the most generous allowance—otherwise, were the carrier to offer a less generous allowance than advertised on one particular segment, the carrier would probably fall afoul of consumer advertising regulations in other jurisdictions.

The ticket number is not indicative of the marketing carrier on the first segment, which is what matters for the DOT regulation. The choice of validating carrier should not affect the baggage calculation, if the flights are the same.