I've also seen this suggested before, often enough and by historians that I'd say it's at least a well known theory. The fact that this article doesn't even mention it makes me a but suspicious that it's biased.

Of course the reality of nation state politics is complex and there's never going to be a single "true" theory.

My suspicion here is that the buffer theory takes agency away from the Thai and makes it seem like just luck, while this article does the opposite, and the reality is probably a big mess somewhere in between those.

If Siam realized that there was value in being a buffer, rather than a warzone, and made the changes necessary to become that buffer so that accepting it as such was better for the colonial powers than trying to take it, that’s not taking away agency; that’s standard politics for small states nestled between major powers.