You cannot help but have other potential readers, though. Me, for instance. You can choose to reply only to Ben, but as it is a public venue, members of the public are free to point out, as I did, when you are not taking particularly good advantage of this.

Furthermore, even if you are replying to Ben, all that's coming out at this point is irritation. You could collapse your comments to "This is all rubbish, as we've discussed before.", and little would be lost.

Finally, it's not actually that hard to make points. For instance, one can point out that because the universe has no goal or agenda, using emotionally pejorative words like "parasite", whose connotations specifically include an exploitative degradation of another's capacity in order to benefit oneself, is a category error. You can do things like this in less space than you used to express irritation. Then if there does happen to be another reader, they can go, "Oh, huh, maybe that's a good point", not merely, "Wow, Spike sure doesn't like Ben."

Since quality public discourse is a virtue overall, I do not think your "I replied to Ben" defense is compelling. (Nor is "we talked for years", nor is "you can guess what I meant".)