Effective Altruism is apolitical in the same way the Southern Baptist Church, Universities, and Black Lives Matter are apolitical.
That is to say none of them are apolitical in the slightest, but they believe it would be nice if everyone just agreed with them and their mission.
EA is neither "left-wing" nor "right-wing" it simply has a set of commitments and priorities that are at odds with the main axis of political conflict in the contemporary west.
In a PR system, I'd imagine a small EA party joining left- or right- coalitions opportunistically.