The foreign corporation owns 50% of the new US Corp that is starting in 2020, and all revenue is US-ECI ...
The US-ECI is that of the domestic C-corp. If the foreign corp has its own US-ECI, that would trigger a filing requirement for 1120F but this may have implications to the applicability of Article 10.
I had read that if a foreign corporation is claiming a reduction in taxes it must file a 1120f rather they owed taxes or not since their dividends is US-ECI?
It is not consistent with Article That's not exactly what it says, you may like to read the instructions again.
... since their dividends is US-ECI?
Are you sure the dividends to be paid to the foreign shareholder are US-ECI? Dividends, as they are defined, are generally FDAP. If the character is really US-ECI, the substance of that may make them ineligible for Article 10.
Still an AllStar