They put the poll on Twitter.
Online polls never measure anything about general sentiment, since the poll respondents are self-selecting. Any online poll on a controversial subject simply turns into a horse race between competing activists. In this case, the progressives mobilized their forces faster. In other cases, the conservatives have gotten their own troops to distort results.
No legitimate poll will produce such a result. People are coming around to the concept of single-payer health care, but about as quickly as tectonic shifting. It will be a while before “medicare for all” enjoys 73% approval, maybe a long while, maybe never.