And then there were ten. Democratic candidates on stage, that is

(Italics mine)
If you dig down to the Reuters/Ipsos poll, it was a simple question: “Would you support or oppose the following? A POLICY OF MEDICARE FOR ALL? (When it comes to the U.S. healthcare system).”
So, it does say that 70% favor MFA. It doesn’t distinguish it from modified ACA or any other plan, and didn’t ask respondants to do so. “Instead of” isn’t there.