This text is also clearly not meant as a treatise of linguistics about Bahasa Indonesia, it's more of a cultural trivia piece so I don't think it's really fair to criticize the lack of cold hard linguistic facts. It's also an important piece of information for anybody considering learning the language, as it turns out that it might not be as useful as one might guess even if you plan to live in Indonesia.
I do challenge your assertion that "claims about the relative simplicity or complexity of different languages are rarely grounded in actual linguistic evidence". It's true that it's very hard to judge the "absolute" difficulty of a language because everybody has very profound cultural and linguistic biases. It's also true that it's not generally a very useful information, I don't know which of Italian or Vietnamese is objectively harder but I know that as a speaker of French and English I'd probably have an easier time learning the former than the latter.
That being said I think if you're familiar enough with the languages you're "rating" you can come up with a pretty decent classification of "absolute" difficulty. For instance I'd personally consider Portuguese to be objectively harder than Spanish, because while they're extremely similar languages Portuguese does have an objectively more complex phonology and slightly trickier grammar. I would also say that French is "absolutely" more difficult than Italian because of the more complex spelling and phonology.
Chinese is widely considered a difficult language, partly because it's tonal (and you could say that's cultural bias) but also because of its very complex writing system. I don't think anybody could say that the differences in difficulty between written Spanish and written Chinese can only be attributed to cultural judgement.