There are degrees. This error isn't like an English speaker discussing French pronouns and commenting on the pronunciation of "moi" and saying it as "moo-ah". Gladwell's error was like saying it as "moy", rhyming with "toy".

The first error is completely understandable. The second one can't help but change one's perception of the writer... if one has any familiarity at all with the topic being discussed. For those who know nothing about it, it would seem like a triviality, much like "misspelling" the word igon values.