It seems this is the language of an oath, as long as you didn’t say it explicitly, and certainly not the language of a vow (because it refers to the person). Right?

It doesn’t matter. But he can forbid the chocolate to himself, and then it’s a vow. Some of the medieval authorities hold that anything involving a prohibition on the object is a vow regardless of the wording. Of course, if he forbids upon himself the act of eating chocolate, that is an oath.

What about “I vow that I won’t eat”? A vow? An oath? Is it permitted to eat?

That’s what I was talking about here. Obviously it’s forbidden to eat; the question is whether that is by the law of a vow or by the law of an oath.
If one vows using the language of an oath, as far as I remember there are also opinions that it does not take effect at all (for example, “I swear that this loaf is forbidden to me,” or “Konam that I will travel to Jerusalem”). But here this is not a vow in the language of an oath; rather, it is wording that is doubtful whether it is a vow or an oath, and either way it is forbidden.

There is a dispute among the medieval authorities whether a vow expressed in the language of an oath takes effect by the law of an implied expression (Nachmanides), or whether it does not take effect at all (Rashba).
Maybe that is what you meant when you said “a Torah-level doubt” (I had remembered only Rashba’s view).
In any case, clearly the proper thing is to appear before three people who will release the vow.

As far as I remember, there is also a view that it takes effect as an oath and not as a vow, so this is not a case of an implied expression. But when I spoke about doubt, I wasn’t referring to that, but rather to wording that can be interpreted either as an oath or as a vow, and that is a different Talmudic topic. It’s not a vow in the language of an oath, but rather possibly an oath and possibly a vow, and that is forbidden either way.