The whole passage to me is clear (I know you disagree and that is ok)
1. There were people who were part of the church, who now have become antichrists (against Christ)
2. These people left the church
3. We (the church) do not need become discouraged, Because they left to prove that they were never part of the church, (now here is the important part) "if they were part of us (the church), they never would have left"
That last part.. If they were a part of us,, would have to include all in all time,, or it makes no sense.. Because john was not talking about a few people, he spoke of a whole group of people (people who had been a part. and left. and now deny Christ)
so, yes, To me it does say this, I can not see it any other way.
and no. I take the whole context, not just one verse (vs 19), One verse to make a doctrine would be foolish..