I don’t understand why Hexbears think that voting for Biden means supporting his policies.

If Biden was some feckless loser who just kept bumbling around the White House as a well meaning but ineffective in stopping the fascism the USA is based on/infected with, you’d probably be able to make a “critical support” argument to vote for Mr. Magoo to the Hexbear crowd. But this goes beyond anything that could justifiably be argued as “critical support”.