Sorry if that sounded like splitting hairs but I thought it bore on the subject. Anyway I found some comments by B. Ehrman interesting. He takes the position that 1 JOhn was connecxted with the Johannine community and that the remarks at 2:19 require then that the separationists have been formerly associated and alingned with them. This would seem to make the Jewish Christian angle unlikely. Rather he proposes that the Johannine separationist had evolved a higher Christology that presaged formal Docetism.
The "come in the flesh" expression seems to mean just what it says rather than be idiomatic. Ignatius used the very expression precisely to refute Docetic notions. (Ign Smyrn 1:1-2, 3:1, 4:2, Trall 9:1-2) The author of 1 John is consistant in declaring the physical nature of Jesus by opening the book with assertions that the Christ could be seen heard and touched.(1:1-3) Furthur the the formula that insisted Jesus Christ came "in water and BLOOD" seems to be consistant with his theme of refuting Docetic christololgy.
So Erhman takes the opinion that the work specifically was a Johannine community effort to reign in Docetic ideas that bloomed from within the group. Separationists likewise were indirectly refuted but the object does not seem to be them. Nor does the idea that they were combatting a Jewish revival within the group seem evident.
Even if the book was not from the Johannine community at all (my earlier statement) but Proto Catholic/Catholic most of this argumentation still seems to hold water.