I'm also totally not convinced by this argument.
Synthetic data as an input to a careful training regimen will result in better outputs, not worse, because you're still subjecting the model to optimization and new information. Over time you can pull out the worse performing (original and synthetic) training data. That careful curation is the part that makes the difference.
It's like DNA in the chemical soup. It's been replicating polymers since the beginning, but in the end intelligence arises. It didn't need magical ingredients. When you climb a gradient, it typically takes you somewhere better.