I feel like it's the opposite. Most people who speak languages with complex grammar natively can not clearly explain the grammar to you, because they use the correct grammar intuitively, and they have learned to do so by having a ton of input in that language.
This is a bad example because it's probably more wordy/complex than it needs to be but I couldn't begin to name the various grammar being used in: "I would not have gone to Paris except that a friend decided to give me a free ticket."
Good luck getting a 3 year old Finnish person lecturing you on Finnish grammar - Even though the kid can easily ask for a ice cream in both past, present, and future.
