The definition of "liberal" being used here by The Hill is "voting with the Democratic Party". Their definitional left end of the spectrum is "bills put up by the Democrats" and definitional right end of the spectrum is "bills put up by the Republicans". These are not actually meaningfully "liberal" and "conservative" as the terms are used elsewhere.

Harris is a party-line voter (pretty obviously, as an insider she's defining the party line in the first place). The Democratic Party isn't leftist and nor is Harris. It's routine in most democracies for elected representatives to be party-line voters.