I read the article and then listened to the whistles. Surprisingly (since the article said otherwise), they were rather intelligible. They don't use one whistle per letter, which would have been weird, because you don't just say letters when talking (like "aitch ee ell ell oh", you just say "hello").

Similarly, this just seems to be taking the cadence of speech and whistling it. I listened to the proverb whistle, and it "clearly" says "η ζωή είναι πολύ ωραία όταν μπορείς να είσαι αυτάρκης", which I definitely wouldn't have gotten if I didn't know what they wanted to say, but which is different enough from the English translation to show that I could actually derive information from it.

Overall, I don't think you can actually hold a conversation in it, it's basically like taking speech and encoding it in low-bitrate audio. Lots of nuance gets lost and you're left with the low-entropy signals, which are hard to disambiguate unless you know what they're saying.