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You are correct. You are not engaged in a U.S. trade or business. IRC 864(b)(2)
The address itself also does not constitute a U.S. trade or business.
If you were physically present in the U.S. and working then you might be considered engaged in U.S. trade or business. Thus this would be considered Effectively Connected Income "ECI".
You have nothing to worry about by merely holding a U.S. address for your disregarded Delaware LLC.
I trust this provides the clarity you were looking for, please let me know if you have any further questions.