This actually came from experts. Maybe not the same ones working on the specific vaccines but ones who have decades of experience within the vaccine space. Just because you read it in the media does not mean the media came up with it. They don't actually make things up. And 18 months was not worst-case it was average case with the best case in the media being Autumn. Just because the best case came out, doesn't remove the point that we were repeatedly told by experts (WHO) that 18 months was a good case scenario. While other experts said that was super optimistic.
It was the average case for a once-in-a-generation vaccine development process with unlimited resources, occurring in 2020, with ample research done on similar virus mere years prior?
If not, then it sounds like a useless "average" that was picked to fill air time and not to represent the best data we had at the time. And who had the best data at the time? The vaccine teams that had already developed the vaccines that are now being injected into the arms of Americans today. But they don't trot out on TV, so they had to grab some filler scientists to pretend they knew what they were talking about.
We were told a lot of things by the media, nearly all of it oriented towards pessimistic outlooks, worst-case scenarios and often just complete misrepresentation of the science. If you asked the people working on these vaccines back in May, they would have told you 18 months was some made up nonsense.