You probably know this, but you you claimed it was "persuasive evidence", so I'll play along. In a poll where you are asked to choose which of two teams you are rooting for, and "neither" is not included in the results, this doesn't accurately represent "the entire fanbase" at all. All it represents is which of the two available choices people are backing. They may or may not normally be fans of either team, but being asked to select from a binary situation, they chose.

And the poll is already somewhat self-selecting. A higher concentration of fans who actually visit the ESPN page and take the poll will be fans of one team or the other. So, again, not indicative of anything about "the entire fanbase".