Let me try to explain it in steps.
1) If someone (in this case, Someone) knows in advance with absolute, 100% certainty, what I will do, that means, by definition, that it is not possible that I will do otherwise.
2) If it is not possible that I will do otherwise, that means that I don’t have a choice.
3) No choice = no free will.
The l’maalah min hazman approach makes it analogous to someone seeing what choice was (focus on the past tense) made, which does not affect the original choice.