Why is are people so shocked and aghast at this quote? She's not wrong. In most other functioning representative democracies, usually parliamentary systems, minor parties have an actual presence in government.

AOC and Bernie have a very different ideology than Biden and Buttigieg (though I'd argue Buttigieg doesn't have an ideology outside "say whatever gives me power over others"). AOC would in the NDP while Biden would be in the Liberal party, as an analogy. AOC would be labour, Biden LibDem.

Our two party system doesn't mean there's only two ideologies. Instead different, sometimes opposing, ideologies have to exist within the same party.