Not every applicable route is being cleared for DOO(P) all at once. For those routes which are being cleared for DOO(P), for stock with bodyside cameras, DOO tests are taking place on a "rolling programme" basis. Some of the test runs are quite extensive in terms of time taken, and I'm pretty sure it would be impractical to have a sudden "blitz" of staff and trains doing the testing, taking up a lot of pathing space and workforce hours. Testing covers a number of different items, but if you notice those schedules (available in RTT if you can find them!) where they are stopping trains for around 5-15mins at each station, these are principally devised for checking that all bodyside cameras should be able to be used under different lighting conditions. This testing sometimes has to be done numerous times, to check any subsequent improvements made as required to the lighting equipment, or because bodyside cameras on different trains can be quite variable (!), or varying according to day/night timing.
I am somewhat surprised to see they have been happening on ASLEF strike days, as up until now, I am fairly reliably informed that ASLEF union reps have apparently been released to accompany pretty much all such test trains, including special night shifts for testing camera performance in the dark. Either ASLEF reps have been breaking strike, or - and this is rather more likely - they have been dispensed with, with the new mode of operation to be introduced after testing by GTR representatives only. I can only speculate on this, but perhaps GTR have decided that as they cannot agree with ASLEF about extending DOO(P), there is no obligation for them to stick to any agreement about their reps being present during the testing programme either, or only on days when they book on as normal.
It should also be pointed out that Southern have been running empty trains between depots and sidings to balance stock, which would be irrelevant movements for passengers anyway.