If an instance isn't defederated with another instance, it can talk to it.
You can see which instances an instance defederates with yourself. For lemmy instances, it's at /instances. Just check each end.
So, for example, I'm on lemmy.today. http://lemmy.today/instances doesn't have lemmygrad.ml in its Blocked Instances list (it doesn't defederate from anything, as a matter of policy, in fact).
https://lemmygrad.ml/instances doesn't have lemmy.today in its Blocked Instance, so it isn't defederated on their end either.
Ergo, they can communicate.
Pretty easy to check a pair of Lemmy instances for that.
All that being said, though, if you want to create a series of throwaway accounts to argue with them without them banning you, I think that both you and they are going to be happier if you two stay away from each other. It's just not worth your time, and I think that the chances of there being a productive outcome for you or them isn't very high.