"Estou fechado com Bolsonaro e não abro."
You're right, it's not to be taken literally. "Estou fechado" is a slang phrase for "I totally agree with", "I'm committed to", or "I'm wholeheartedly dedicated to".
It's often followed by "e não abro", also a slang phrase meaning "and I won't change my mind", "I won't rethinkrethink", "won't reassess".
"ok, estamos fechados" = "ok, agreed."