I don't see any assertions of "how they got it wrong" in the article, at least about the class sizes. The article states that the number was wrong, that it was obviously wrong, and investigates how the mistake was "corrected". The article is not concerned with "how they got it wrong" at all, only "how they corrected it".

The obviousness of the error is brought up because it's relevant to these bullet points: "Both correction notices are notably vague as to how bad the errors were" and "They don’t seem to have reviewed the teacher strike piece for other mistakes".