You can tell how vague the grammar complaint was it probably wasnt a big of a gaffe that they think it was. Also, slight tangent, but the part where he said that it admitted where there was an error but was not able to say why it made the error is one of the issues with model sycophancy; it want to validate the users feelings of being wronged, but also does not want to say something factually incorrect. So it produces this fail state of saying sorry for nothing, but it cannot backtrack.