Interesting...but I just looked up the family line and something doesn’t add up to make WC a presumptive heir to the dukedom...
The 8th Duke (WC’s dad’s elder bro) became Duke in 1883 and his heir was born in 1871...WC was born in 1874 so there was never a time that the 8th Duke didn’t have his heir present...and that heir became the 9th in due time.
The 10th Duke was born 24 years after Winston.
Before the Vanderbilt Duke was born Winston was the presumptive heir.
I am not sure why you don’t understand this but maybe because you don’t understand presumptive heir term as it relates to Peerage. 9th Duke doesn’t have an heir - it goes collateral which was Winston Churchill for 24 years.
Here it is
https://en.wikipedia.org/wiki/Heir_presumptive