Your general point is valid, but as they were going to East Croydon (or Streatham Common) they most likely changed at Wimbledon for Tramlink or went via Waterloo and London Bridge.
Southern Diverted services from Horsham via Dorking or from Dorking Only to London Bridge don't go through East Croydon and nor dis the trains on this line when they terminated at London Victoria. Instead they travel via Leathehead, Ashtead and Epsom, Sutton, Ewell East, Carshalton, Hackbridge, Mitcham Junction and Mitcham Eastfields etc (and the last service back of the night only at Balham) and then run non stop to London Bridge just as they previously ran non stop to London Victoria.
So passengers on lines from Horsham to London via Dorking and Epsom were never going through Purley, East Croydon etc previously and nor are they travelling through those stations now. So they were never most likely to change at Wimbledon since Southern trains through Sutton have never ever called there and nor do they now during the near total blockade on services in to London Victoria. Only South Western trains from Dorking to London Waterloo call at Wimbledon but they don't call at East Croydon either.
This is because surprisingly enough Southern does actually run quite a few lines out of London Victoria that are not services involving the Brighton Main Line but customers who use the Brighton Main Line services and also many Southern managers in train planning quite frequently appear as though they seem to be unaware of the existence of those other Southern train lines, although I think most of them are well aware of Southern's East and West Coastway lines............