This myth was started by Robert Sherwood in his book "Roosevelt and Hopkins". Per Sherwood, FDR traveled to Hawaii in July 1944 and met with MacArthur and Nimitz. After talking to both men, he was swayed by MacArthur's oratory and decided to invade the Philippines in October 1944.
This is false. FDR came to Hawaii on July 26, 1944 and left three days later. He spent the night of the 27th and the morning of the 28th talking with Mac and Nimitz about the future of the war in the Pacific. The rest of the time was spent inspecting the troops, making speeches, touring Hawaii and various entertainments. MacArthur had no staff with him and spoke off the cuff.
The decision to invade the Philippines in October 1944 was made by the JCS in September 1944. The final decision to invade Luzon and liberate Manila was made by the JCS on October 3, 1944.
So, FDR did NOT decide the strategy of Pacific war in July 1944. The decision to invade the Philippines was NOT a political decision. It was military one made by the JCS, Marshall, Arnold, King, and Leahy.
BTW, there were two reasons for FDR's trip. One was to shore up his popularity by showing him as a war leader. The second, was to provide FDR, a very sick man, with a 4 week vacation from DC.