I don't think they dictated it at all. If I remember correctly, the original ending was largely the same, but it didn't include a (large, dramatic) prop. The execs told them "is this the best you can do?" and Vince Gilligan thought about it with his writers, and one of them suggested adding said prop.
Honestly, when I saw it, I wasn't sure it was actually what the main character said it was. It wasn't that big of a deal and it didn't meaningfully alter the trajectory of the story. It did make the finale have more of a "cliffhanger" feel to it, although cliffhanger isn't really the right word. More of a "wtf" moment.
