The op-ed is here. Boiled down, a New York opinion says lawyers shouldn't look for metadata sent from opposing counsel (outside the discovery context, obviously), while the ABA says you can. Sounds like a technical disagreement, but it boils down to whether it is "dishonest" to look for metadata.
What I have heard smart lawyers do is plant false metadata to mislead those who would be dishonest.... wait, there's something strange about that solution.