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@anepcar wrote:

The foreign corporation owns 50% of the new US Corp that is starting in 2020, and all revenue is US-ECI ...

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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.

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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?

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It is not consistent with Article That's not exactly what it says, you may like to read the instructions again.

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... since their dividends is US-ECI?

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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.