No, the Saturday service is the subject of the complaint: local pressure succeeded in getting SX evening trains restored, and that is what the OP wants - rightly.
The justification would be that people working in London (etc) often stay late in town after work, and need/want trains home. Mid-evening trains out of London seem to be much better used than in my (and your?) day. (Of course, while WFH is mandated, this is likely to be different.
Agreed that is indeed my issue.
The reason that the lack of a Saturday evening train service has become pertinent is because it is now being used as a weekday timetable (allegedly for the time being but that time being might stretch on to May or beyond), although that basic Saturday timetable has already been amended on weekdays to bring in to service a train around 6am that normally runs up to London Victoria out of service as a non passenger train.
In my opinion Southern should have also taken that step with a late evening train on the Saturday timetable that runs in passenger service to Dorking and then out of passenger service to Horsham by keeping the train in passenger service to Horsham
However the normal Saturday service has no train back to Horsham via Dorking after 5.25pm so I would say that as a minimum a train through to Horsham at 7.25pm and the conversion of the 11.26pm train that terminates in Dorking and runs empty to Horsham in to passenger service is the minimum requirement needed even on this short temporary timetable. Between 2004 and 2018 there was only a 2326 service from London Victoria to Horsham via Dorking and this was a train that prior to 2004 still through ran to Horsham but terminated for passenger service at Dorking.
The practice of running trains out of passenger service from Dorking to Horsham is in my opinion both unjustified, inconsiderate and extremely wasteful of limited public transport resources.
On Saturdays they are much less likely to trek up to town for a show, or if they do, they may well drive further into town rather than plan around the infrequent service to the nearest station.
I agree that in the past people may have driven closer to town or in my case even all the way in to town because of no evening train services south of Dorking on Sunday and noting that the cost of an Off Peak train ticket from Ockley to London is the same as Dorking Off Peak but that is not much use if no train service exists at all when one needs to travel.
However with the introduction of a ULEZ that impacts many existing vehicles covering the whole area inside the North and South Circular 24/7 and the Congestion Charge still remaining in force from 12pm to 6pm on Saturday and Sunday (after a change from the recent 7am to 10pm on Saturday and Sunday) everything has changed so that it is now much more attractive to take the train rather than drive in to London on Satuday and Sunday but so far Southern's pattern of train services south of Dorking on Saturday (no evening service) and Sunday (no service all day at all) has not caught up with this changed state of affairs with the 24/7 ULEZ and 12pm to 6pm on Saturday and Sunday Congestion Charge (this appears to be a permanent long term arrangement) now encouraging many more people to undertake their entire journey to London on public transport.