Because it's a straight up Trump-esque semantic dodge. Shes knows what 'democrat' means in the context of the two party system, modern politics, and an election cycle, and so does everyone else. But there she is, clearly implying that Bernie isn't someone that left-leaning Americans should be voting for, while leaving the door open for, "What? I never said THAT!"
"I said 'their' not 'they're'! Don't you people know what the Oxford comma is?!?!"
'Petty' is a good word for it.
You're really reading a lot into a question that was specifically about Bernie's support of down party tickets in the Democratic establishment.
They weren't asking about his ideology, they were asked about his party affiliation.