The assumption might in fact be wrong. The main thing I'm refuting is Ayyadurai's assertion that a "flat line" is the only natural thing to expect.
In the video he keeps saying that this "beautiful sloping line" is something super unnatural, but here is a perfectly natural explanation.
If instead you had Republicans more likely to vote Trump than downballot, with a difference greater than what you see among Democrats, you'd see an upward sloping line.
The line is a function of voter-preferences--the only way it'd be flat is if each party had the SAME preference for Trump relative to Republican downballot votes.