Did they "endorse" him? I read the link and it didn't sound like an endorsement. Also, even if they did, I don't think it's fair to dismiss the whole party as revisionist just because of one incident. CPUSA is more worthy of being called revisionist than FRSO.
So, cheering on a Democrat, that conflated (antizionism with antisemitism) as if he's going to support the Working class is apparently not opportunism? Literally, from the source text:
Chicago, IL - On Tuesday April 4, Brandon Johnson won 51.4% of vote in the Chicago mayoral runoff election. Johnson's victory is also a victory for working and oppressed people, as demonstrated by the range of organizations and individuals who celebrated with Johnson and his family at the Marriott Marquis on election night.
— https://www.fightbacknews.org/2023/4/6/brandon-johnson-elected-mayor-chicago
(Pro tip: You can't support the working class through bourgeois democracy.)