Sistani's decision to not engage in or lend his support to the Iraqi Shia insurgency, the Mahdi army and parliament was a political decision.
I'll just add in that I don't think this is really accurate. Sistani was instrumental in forming the UIA in 2005 and they used his face on their posters. He not only vetoed the CPA's original plans for Iraqi elections in 2004-2005 (intended as a sort of caucus process), but outlined exactly what form legitimate elections and constitution-writing should take. He specifically did not lend any legitimating authority to the CPA in this process, instead selecting the UN as the proper overseeing authority.

Sistani has repeatedly indicated that the clerisy should take an overseeing role with regards to state affairs, and indeed this is the role he has arrogated for himself in Iraq. But he does not endorse Iranian style institutionalization of this i.e. with the office of Supreme Leader, Guardian Council, etc. But he certainly not a pure apolitical "quietist", a title that better fits the al-Khoei family. Reider Visser has this excellent paper about Sistani's views on politics.