My mistake on E&C, sorry, I was relying on memory
But suggesting that there is spare capacity at Waterloo means that there must be spare capacity from Clapham Jn is a brave leap n logic. Waterloo, of course, has many more options for onward travel - Jubilee in both directions, Bakerloo and the drain, plus countless buses for a short trip to various locations in Central London, main line from Waterloo East, and of course walking. Passengers at Clapham Junction heading for Northern Line destinations have none of those, realistically.
As I said, I’ve seen the modelling, and it is very clear.