If you read the quoted pasuk carefully, you'll see that it isn't actually teaching that man has free will as if that were something that the Torah needs to teach. It's merely saying that, given that man has the freedom to choose, I'm telling you that the decisions that you make are ones of life and death, blessing and curse - so choose the path of life!
The reason why this verse is brought as a proof that, according to the Torah, man has free will, is merely because some people question (or even deny) its existence. Thus, this verse is used to show that the Torah was working with the background assumption that man has free will.
Regarding the Ramchal's opinion: in Derech Hashem (I, ch. 5) he does write that some decisions may be 'forced' מצד גזרה עליונה לשכרו או לענשו, in order to reward or punish him. It seems like those actions, though, would be recognizable to the person as actions that don't originate in his will, but from external forces (like a reflex). It could be that this is the Ramchal's intention in the continuation of that paragraph:
במה שהוא נמשך אחר הגזרה שעליו, יהיה משפטו כשאר עניני העולם, שתנועתם מלמעלה למטה, כפי מה שיניעום הכחות העליונים. ובמה שמצד בחירתו, תהיה תנועתו מלמטה למעלה
The Ramchal also writes (Derech Chochmah 12) that some actions of a person aren't a result of free will, but of מה שטבעו מכריחו, או קיבוצו המדיני - his nature, or his societal/political surroundings. I'm not entirely sure how one would be able to tell this from acting upon other desires according to the Ramchal, but it could be that he'd agree with Rav Dessler's explanations in his many discussions of free will in the books Michtav Me'eliyahu. In a few places he writes that a person's point of free will is where he recognizes what he's supposed to do, but has a perfectly equally strong desire to do what he knows is wrong as he does to do what's right. In what's probably the most succinct formulation (vol. 5 pg 500): בחירה אמיתית מתרחשת רק כשהאמת שהוא מכיר וכח יצרו מתנגשים בקרבו שוה בשוה