But both players/mechs may fall under that definition that turn. So here's two situations, let me know if my thinking is correct.
We have player A with mech A, and player B with mech B. Player A has won initiative. Mech A and mech B stand in a way that their LOS falls perfectly between hexes, one of which is blocking LOS.
Situation 1:
Both mechs shoot at each other. In this case defender is the one that is shot at first - mech A, because player A has won initiative. Player A decides that the LOS is blocked, so neither mech will shoot at each other this turn.
Situation 2:
Mech B goes first because player B has lost initiative. But for whatever reason, Mech B does not shoot at mech A (maybe it has no damage in that range or targets something else). So mech A never was the target/defender this turn. Then player A is resolving attacks. Mech A shoots at mech B. Mech B becomes the defender as defined on page 69, so player B becomes the decision-maker about the LOS.
So I guess the intention in this case is that defender is the first mech that gets targeted this turn on that line between hexes?