They could literally just have 3 interns make hundreds of fake accounts on various instances, and flood the network with low-grade but ultimately harmless content.
My guess is that it is either that, or else bribing people who have existing positions of trust on the network (admins or moderators or powerusers) to undertake some kind of destructive action.
The first is cheaper and much more effective (and won't get detected instantly), the second is more in line with reddit-exec-brained thinking. So kind of a toss-up. Something along the lines of those ideas would be my guess though. I thought about some kind of "embrace, extend, extinguish" strategy like Facebook with Threads, but I think even Reddit isn't stupid enough to try something like that (which would only result in a massive increase in the exodus of users from Reddit to Lemmy).
Edit: Well... something occurred to me. "Low-grade but ultimately harmless content" is exactly what they have been pushing Reddit towards, because they think it is better because it traps people in dopamine loops more effectively. Flooding Lemmy with that stuff (more so than it already is organically) would fuck it up from my point of view, but maybe from their point of view, something like really emotionally toxic random hatred in all directions. Maybe. Anyway, I think flooding Lemmy with content that turns people off via sockpuppet accounts is probably the easiest and most effective way, and I don't see much way to prevent it.