There is no lie part in that one sentence, Joseie you are right, but looking beyond being pedantic within a single sentence, the lie is within the full paragraph made up of several sentences that it would be good for them, it was not good for them because they ended up dieing no longer glorious bodies in the garden of eden, which is why we all die, the only remedy is the truth of christ, that if we repent and turn to him we can be saved from the death and have eternal life like the Adam and Eve restored, which was Gods plan all along before the deceit. If he had said you will die and be cast out of this beautiful garden then would they have chosen … the lie is also in what he the serpent, failed to say - the most important aspects such as death. But you are right about the single sentence.
An analogy - A parent says to their five year jump into those rapids on a hot day they will cool you down! it is not good for the child (sorry not good analogy, couldn’t think of one off hand for adult… but hope you get my drift).
Thanks for your opening question, these things make us think a little more sometimes God Bless.