Whether they might be affected by the SLOW Debuff or not, it is still a Debuff, and there are other things that trigger (or increase, etc) based on (any) Debuffs on someone.Do those 4 champs (3 shown + BWCV) have abilities that rely on opponent Debuffs ? (or maybe Quest Map nodes that do)
The Prowler+Spiderpunk one has got to be up there