You didn’t address the falsehoods spread by those that left their group. You commented on those that didn’t accept Jesus as G-d. Is that the falsehood that you think constitutes the spirit of antichrist? If so, which former members do you think John is speaking of?
Anyway, the "anyone" in the above paragraph includes everyone, past, present, and future.
And, if those who "left the group" were engaging in the behaviors in the above paragraph, then they were "antichrists", or, to make it easier, "against Christ".