Yes, because it sends a message to the users of Forum X that “Bob” and “Dave” are in the out-group for attacking the users of Forum X in such a way, and that this behavior is to be shunned instead of encouraged.
Theoretically (I do not forsee this happening under the current English-speaking political climate, but let’s go into hypotheticals for a bit) disgruntled users of Forum X targetted by Bob and Dave could start using the same tactic of screenshot dunking on a sympathetic Forum Z. Now you have two copies of bad behavior, and Forum X is caught in the crossfire. Potential new users of the forum see these screenshot dunks, and think, “wow, Forum X is full of bad people huh”, and avoid joining. Now you’ve depressed your flow of new users, even though none of this activity is taking place on Forum X itself.
If your flow of new users is being depressed from an undesirable source, well, that makes perfect sense, huh? Community member selection working just fine. If “Bob” and “Dave” remain members of your community, then people who see this screenshot dunking may think “seems fun, I’ll join in too!”.
You should prepare a public statement regarding the bans before they are carried out, as it sounds like there is already significant awareness of this incident among your forum members.