From a brief look at their "people" page, it seems like they're all involved with chapters of the DSA (Democratic Socialists of America). If anything, this suggests against foul play, as their incentive would be to denigrate for-profit healthcare and overstate support for Mangione.

I am wary of the accuracy of the poll because it was an online one, but it does generally agree with a similar Economist/YouGov poll.