I think your understanding is very good, and I'm not one to question your motives, as I know you just want to understand better. We all do.
I'm not an expert, but my understanding is that they have cells growing in a dish, and then they add things to it. For pre-treatment, they add the "treatment" brilacidin to the dish before they add in the virus to infect the cells. They likewise add in the control (water or DSMO carrier), and compare what happens. If at the end of experiment control has 100 viral particles, then 97% reduction from pre-treatment with brilacidin means it only has 3 particles.
Same with post-treatment. They add in the virus to cells first, then 1 or however many hours later add in the treatment brilacidin or control, and compare how many viral particles at some period of time afterwards.
It's possible for pre-treatment they add in the brilacidin or control to virus solution before adding to cells. I don't know, but would seem unusual to me. The pre-print paper when it comes out will describe all the details.