Well I didn't know that Siam was that big but for how long did it control that territory? Also my argument is not irrelevant and France doesn't control any territory in Africa. The point is that Polynesia is such a small and unimportant area that the French were and are capable of controlling it unlike the former African and Indochinese territories.
France = 670000sq/km
Polynesia + NZ = 1400000sq/km
I'm using Polynesia=Pacific islands due to the Micro/Mela/Poly divisions being a rather wonky product of 18-19th ethnography. And that is of course, just the landmass. The Pacific ocean and area in total is kinda...big.
I've said it before and I'll say it again. Civilization as a game requires a combination and balance of actual history, the 'what happened', along with the 'what if'. If you're going to focus solely on the former, or even worse run around being grindingly eurocentric, why not just pick up a history book.
The idea of playing through the game and say having France beat out Britain for the mastery of the globe is equivalent to playing a game where Carthage beats Rome, and that is again, equally equivalent to one where Polynesians conquer the whole damn world.
Similarly, I tire of the same defective arguments against inclusion of various civilizations. 'They're in the stone age' for example has a logical conclusion (in that line of thought) of including only modern nations. France > Polynesian's is the same as New Zealand > Classical Greece - New Zealand's military (yes, we have one) could conquer them. But it is also a self-defeating argument considering that all Civilizations in the game
start in the stone age anyway.
Civilization as one of its core gameplay elements effectively takes the history of mankind, and then tosses all its technological, cultural, religious, military achievements up in the air in a big lolly scramble. At this point who actually did what becomes utterly irrelevant. Persia had a big empire, Babylon good at maths? Fantastic. Playing as the Mali you could conquer a bigger empire, and lead the world technologically instead. It assumes that technological progress would be more or less the same in a rerun of the world. Similarly, buildings, governmental process (civics) catering to human needs would be the same. Wonders are a bit more historically specific, but you could still envisage them as more abstract 'great accomplishments'. Subsequently, there is no reason one of those peoples cannot be something a bit further afield than the staples of world history. Babylon building the Pyramids first is really no different than Polynesia building them first.
Long story short, Firaxis is quite justified in choosing Civs from around the world, in this case possibly Siam, in another ideal case Polynesia.