Telling someone who speaks a language at an intermediate level that they don't always speak idiomatically is pointless - they already know that.
Sure, but do they know when they're saying something awkward? Recently I struggled in both England and Barcelona to get carbonated water. In San Francisco I'd be saying "soda water". In New York, "seltzer water". In England, apparently, sparkling water is the trick. I'm a native English speaker so it was fairly easy for me to pick up on that.
Meanwhile my Spanish is not great, but enough to sound like I speak Spanish fluently and make casual conversation and order things at a bar. In Havana I learned "agua sin gas" and "agua gaseosa". I tried that a few times in Barcelona (a city you can navigate entirely in English) recently and eventually got a weird look. Next time I asked and was told "agua con gas". Were it not for the weird looks I'd've stuck with awkward phrasing.
I generally tiptoe pretty gently with non-native English speakers, but have one friend who is insistent upon having their mistakes and awkward phrasing corrected. They're also the one friend who seems most like a native English speaker with written communication (the accent is a whole other story).
If you don't know when you're doing something awkwardly you won't ever improve.
My friend, randomly, said that I speak non-idiomatically / kinda awkwardly in general.
Great. What am I supposed to do with this information? I already know that. It's just frustrating, especially since at the time I was in a rut about my progress.
I already know where my skill level is. I know that I make certain mistakes and definitely don't speak idiomatically.
Telling me this is not only useless but a stark reminder of where I'm at. If you're in a rut, or the accursed "intermediate plateau", it really sucks to hear things like this, even if they're 100% accurate.
Sure, I find myself doing the same thing with the Spanish language — especially in a country with very different slang than the US. Even in English. Even in America. Think about how many ways there are to say soda. They'll almost all be understood but some will stick out more than others depending on the context.
I would never think of saying something that to any ESL learner.
Perhaps you should, how else would they learn?