You are right, this was due to another statement by Trump. But Trump saying something about a vaccine had that big of an effect. As soon as Trump started talking about getting a vaccine out to the people Democrats started to think that an FDA approved vaccine would be a bad thing while Republicans started thinking it was a good thing. As you see in the graph after that statement both groups were almost equally willing to get vaccinated.

> Democrats' reduced confidence follows President Donald Trump's Labor Day announcement that a coronavirus vaccine could be ready in October, as well as subsequent news reports stating that Trump is eager to see a vaccine delivered before the election. Trump's accelerated timeline does not align with that of many government health experts, and this disagreement has raised concerns as to whether a vaccine distributed that soon would be effective and safe.

https://news.gallup.com/poll/321839/readiness-covid-vaccine-...