That sounds like a viewpoint that would be vulnerable to just accepting everything that is as preordained. Every country's destiny is heavily influenced by their neighbours.

It is by no means a given that Britain and France would see a need for a neutral buffer zone given that the home countries of the two powers only had a small channel separating them in Europe. They'd be used to the idea of sharing a border. We can all but guarantee that both of them had people looking at the map every so often and asking if it made sense to officially add Siam to the imperial holdings.