Based on IATA rules, DL's rules don't necessarily govern -- they just get to decide whose policies apply.
Starting or ending in the USA or Canada: If your single-ticketed journey starts or ends in the USA or Canada (this does not include journeys that only pass through the USA or Canada), the 'first marketing carrier' decides the allowance and charges that will apply for the entire journey. They can apply their airline's rules or those of the 'most significant carrier'.
In this case, I think under standard MSC rules, whichever airline is marketing the LAX-MEX leg (regardless of who it's operated by) would be the most significant carrier as that'd be the first international sector. However, standard MSC rules don't apply here due to the USA rule that always defers to the marketing carrier for the first leg.