This is a tricky one. While yeast does contain glutamate, so do plenty of other foods with protein, like meat. The glutamate isn't the issue - it's when these proteins are hydrolyzed at high temperature, creating a different form of glutamate (a less healthy form).
I've read both sides, and I'm not convinced that yeast extract contains the unhealthy form of MSG - which is fundamentally, chemically different from glutamic acid. But I'm also not convinced that people who are very sensitive to MSG wouldn't have a reaction when exposed to this particular form.
We say nutritional yeast is okay for the Whole30, and I doubt that yeast extract is going to present a problem for 99.99% of folks doing the program. But for now, as Tom said, we'll encourage you to look for foods and products without these added ingredients, or better yet - just make your own chicken broth. However, I would see no need to start over because you ate some yeast extract in your soup.