It's an interesting question as to how Phibun and the Thai military would have responded if Japan had not been able to offer the face saving prospect of giving them a share of the conquests.
Well, Japan had already helped them gain some Cambodian and Laotian territory at French expense, through its friendly mediation.
Now if the Japanese are moving to occupy Thailand without simultaneous attacking the British, then yes, they cannot immediately or openly promise the Thai territory from Burma or Malaya to help Phibun save face at that moment.
On the other hand, the Japanese could try to sweeten the deal for the Thai by offering them even *more* of Laos and Cambodia in return for alliance and military basing rights.
Don't forget that from 1942 on while officially allied with Japan, the Thai government was still divided into pro-japanese and pro-allied camps. Pridi Bamonyong, who refused to sign the declaration of war on the United States formed the Free Thai Movement which grew to over 90000 strong and engaged in intelligence gathering, sabotage, and rescuing allied airmen.
Also on a side note, the Thai government was so divided that the Thai ambassador to the USA refused to deliver the declaration of war, which in return caused the United States not to DoW on Thailand. The situation there was... complex, to say the least!
All true and a good point.
However, this just seems like a wise move by a small, weak power. Keep a foot in both camps (like the Scottish clans sending a son to fight on both sides of a rebellion so one will always be on the winning side and able to advocate for the family, from
Master of the Ballantrae). From where the Thai Ambassador sat, the U.S., his decision made total sense, America's power and long-term advantages were right in front of him. For Phibun in Bangkok he is looking at a lot of Japanese right in front of him with Americans nowhere to be seen on the ground. Sure, the Free Thai movement emerged and grew. But its growth tracked tio a great extent the turning of the tide in the war, especially in Burma.