Believe Microsoft, believe us... but verify.
I assume (but I haven't tried nor do I want to touch any of these) that you could verify using novel sequences, e.g. type in what you assume is isolated "This is a sentence and..." then try completion. You might get a result e.g. "is grammatically correct" or "I like potatoes". You can try that few times just to get some sample. Try then to try in the other context you assume might actually not be isolate, e.g. your browser "This is a sentence and noise is blue." and repeat that several times. If "noise is blue" never ever appeared before and it now does then you can safely assume that regardless of what Microsoft said, there is actually no isolation.
TL;DR: trust but verify. Yourself. Now.