Let's consider what the sentence would look like without the object pronoun "lo":
Eso decide la embajadora.
When the listener hears the beginning of that sentence, "Eso decide," it gives the impression that the sentence is going to be something like "That decides it [for me]," or "That clinches it." That would give the wrong impression, though, because that's not what this sentence is supposed to be about. The listener would have to switch gears midstream if the sentence were to start out as "Eso decide...."
We can avoid getting started on the wrong foot if we stick the object pronoun in there. With "lo," we know that "eso" is not going to be the subject of the sentence. We know that there is a subject coming, that we haven't heard yet.
You are right that sometimes a redundant object pronoun is included for emphasis (example). But I don't think that's what's going on here.