> It must be normally issuing rulings written by people who aren't the justices themselves.

It's right there in the text:

> the 11 justices and the attorneys who work for them issued 505,000 rulings

> it appears to answer to nobody, nor follow any normal judicial procedure (being both accuser and judge in one body)

That's not the case. The STF never accuses, they only judge. Accusations come from other institutions. The Supreme Court then orders investigations and act as judges.

> What checks on their power do they recognize?

Mostly the Parliament and the Senate, who can at any time pass new laws, including amendments to the Constitution.