It’s easy to confuse the hypothesis of the universe beginning to exist with a visualized line coming to end in a point, maybe with a helpful caption bubble that reads “universe starts here”. But the difference between them is just what is problematic for the Kalam: the line you imagine is a determination within a homogeneous potentiality, and it is only because of its presence within it that it makes sense to give an account of why it stops or starts at one point and not another. But the universe is not obviously a determination of some homogeneous potentiality. Sure, any finite line has something behind it, which raises the question why there is no line behind the point where it starts. But what is behind a finite universe?
If the universe were ten minutes old, it would make sense to say it was not around 11 minutes ago, but this doesn’t mean there is a physical meaning to “eleven minutes”, and without this physical meaning it makes no sense to ask what happened 10 minutes ago in the sense of describing some transition from an earlier state to a later one.
Say a production of Hamlet starts at 5:00. At 5, everyone in the audience can enter into a world with kings, suicides, gravediggers and melancholic philosophers. So where was the Denmark that the play brings to life at 4:59? The world that comes to be is not some transition from an earlier state. The Denmark that comes to be in fact doesn’t come to be at 5 but in the middle of the night, with a patrol seeing a ghost.
The totality of things need not be finite or infinite. It need not be finite for the reason just given, but this non-finiteness does not make it infinite. To send the universe back forever would make it lack a backward limit, but it still lacks a backward limit even if it does not go back forever, since even in this case it does not share a boundary with something else. The javelin argument does not prove that the universe is infinite, but only that physical magnitudes need not have the same properties as mathematical ones (which is equally true of the assumption that it is a hypersphere).