Wikipedia has plenty of other citations, if you have issues with Harvard:

https://en.wikipedia.org/wiki/Protein_combining#Criticism

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.