> people that have spent decades+ in an English-speaking environment, yet still speak in a stilted manner

Again, the standard you're applying is unreasonable. English doesn't have any One True dialect - it's spoken natively in various ways, some of which sound stilted to you. I went to college with a guy from Malaysia whose speech you'd likely find highly stilted (or I did, anyway), yet English was his first language - the only language he used with his parents, the only language he ever used in school, etc. That is what it is - a person can be fluent or native and still sound stilted to you. You may well sound stilted to them!

This is why the rankings you linked don't use such language - they talk about whether someone can use complex grammatical structures clearly and without effort, and so forth, and when judging someone's fluency that's the sort of standard to apply. Saying they sound stilted just says that their English doesn't sound the way you expect correct English to sound, which is neither here nor there.