It's possible the author's language is poor, it's also possible it's very good but they see "great" as normal and so discount what they know due to imposter syndrome.
I've heard a lot of people apologise for their English after speaking flawlessly.
Dr. Hartshorne also points out that native speakers have exceptional precision. Even someone with 99 percent grammatical accuracy sounds foreign. He guesses that I have about 90 percent accuracy, which shouldn’t feel like failure. “Imagine if you decided you were going to pick up golf in your 30s, and you got to the point where you could keep up in a game with professional players. You’d think that’s actually really good. But for some reason, just being able to keep up in language feels not as impressive.”
Meanwhile adults are often too anxious to even try no matter how proficient they might be. I myself went years without even trying to talk to anyone in Spanish despite living in Mexico. Watching children trying to learn a language was inspiring to me.