Does the Almighty's saying apply to a smoker: "But whoever earns evil and is engulfed by his sin - those are the companions of the Fire; they will abide therein eternally" after I swore not to return to smoking, but then did?

The prohibition of smoking has been previously explained, and that persisting in it makes it a major sin.

As for the verse mentioned in the question, it does not apply to a smoking addict. It does not refer to someone who dies upon a sin (even if it is a major one) without repenting, but rather it refers to a polytheist or someone who comes on the Day of Judgment without a single good deed, as stated by Al-Tabari, Ibn Kathir, and Al-Sa'di.

And if the verse were to be interpreted as referring to someone whose bad deeds outweigh their good deeds, then the fire mentioned is not one of eternal damnation for the people of monotheism. The perpetrator of a major sin who did not repent from it is under the will of Allah, and their ultimate destination is Paradise.

This clarification is not intended to belittle the matter of smoking, but rather to explain the truth, for it is among the abominations and destructive harms.