Comparing this article's description of Siam definitely not getting colonized:

> In the face of overwhelming military force, Chulalongkorn ceded the territory, making a necessary concession to preserve the kingdom as a whole. Siam was forced to cede territory to the French in what is now Laos and Cambodia, creating much of the modern borders of Thailand. The French established a demilitarized zone along the Mekong, and the Siamese had to pay an indemnity of three million francs. European powers readily resorted to gunboat diplomacy but even here, France felt compelled to frame its demands in terms of legitimate Vietnamese territorial claims rather than naked conquest.

> Between 1893 and 1909, Siam would cede around 40 percent of its territory to France and Britain. But the strategic retreat meant Chulalongkorn preserved what mattered most: the political independence of Siam’s core territories.

with its contrasting description of Burma definitely getting colonized:

> Following its defeats in the Anglo-Burmese Wars of 1824–26 and 1852, Burma had been forced to surrender four provinces, pay a crippling £1 million indemnity (about $160 million today), and accept the presence of a British overseer with veto power over the king’s decisions.

...

> When Britain manufactured border disputes to complete Burma’s annexation in 1885, Mindon’s modernization efforts provided no protection against a colonial power that was, by that point, committed to conquest.

It rather seems to me like Siam was about two decades into a process that would usually take a few decades more, but got a respite due to World War I distracting the colonizers. (Maybe an even better counterexample to the article's thesis that adopting the trappings of Western civilization protected against invasion by European powers.)