That doesn't really tally with my experience, albeit I am only a sporadic user on non-match days. I caught the (4-car) 9:26 from Southampton to Brighton last Saturday morning. No seats left after Fareham and so rammed from Worthing onward that first class was declassified. No disruption AFAIK so...
They definitely take account of the fixtures when decided when to do engineering works between Brighton and Falmer. They also need to consider other events when planning engineering works, e.g Pride, the Brighton festival. I suspect in general they try to avoid the Summer months for weekend...