Nope, not necessarily. Philippines have CB with Sabah thru the Sultante of Sulu. USA pursued also the same course back then when Philippines was a colony of USA. Only way Philippines can justify moving to Brunei and Sarawak is if both request help to be liberated. OTL Brunei was having a revolt in this time period. Considering that majority of local population in Sabah back then were people of Sulu, Tausugs etc..
Ok, they have a CB. I am still not sure how that doesn't make them the aggressor. After all, Mussolini had a CB on Ethiopia, didn't stop him from being the aggressor...
The Americans aren't stupid. Britain and Co. are far more important allies in the grander American strategy. I fail to see why they would back aggressive Philippine action against the interests of their most important allies in Europe and the Pacific...