I believe it was the AAIB and the NTSB that concurred with Inmarsat's conclusion about the Southern arc theory.
Malaysia's politician are risk averse when it involves the world's spotlight on them. I highly doubt the Malaysian PM would make such a bold statement (plane ended in South Indian Ocean) if he wasn't 100% sure that the 'westerners' told him they themselves are 100% sure of the Southern arc route.
anyway just my 2c.