It's well known that plants can't/don't provide all essential nutrients without supplementing. B12 is a major one amongst others.
Yet the reverse is true.
Sidenote: I wouldn't put much stock in Harvard school of "nutrition". They have massive conflicts of interest with companies like Monsanto and also numerous problems with their methodology like oversimplification of the issues. https://www.forbes.com/sites/trevorbutterworth/2013/05/27/to...
Summary: Yes, eating solely one food may eventually cause an issue. For instance, eating only rice would eventually cause a lysine deficiency -- on the order of ~88% of required amounts. Eating pretty much any other food that is not lysine-limited would likely be enough to make up the ~12% gap.
Yet the reverse is true.
Sidenote: I wouldn't put much stock in Harvard school of "nutrition". They have massive conflicts of interest with companies like Monsanto and also numerous problems with their methodology like oversimplification of the issues. https://www.forbes.com/sites/trevorbutterworth/2013/05/27/to...