What an odd thing for them to say. "I" is one of the most common words in the English language. I very much doubt even grammar purists would recommend against it. What's next? "'The' is considered harmful"?
The mental trick is to append "I think" to the beginning of every sentence and hope that that works.
Also: using the "I" opens up the argumentum ad hominem, which can focus criticism into uncomfortable territory ("Goat bile shouldn't have been added until after the solution was brought to a boil!", versus "You shouldn't have added goat bile until...." One of those is a bit more likely to be taken personally (albeit by someone with thin skin, perhaps as a result of too much exposure to goat bile)).