Without necessarily endorsing the article's ideas....I took this to be like the diamond-inheritance problem.
If service A feeds both B and C, and they both feed service D, then D can receive an incoherent view of what A did, because nothing forces B and C to keep their stories straight. But B and C can still both be following their own spec perfectly, so there's no bug in any single service. Now it's not clear whose job it is to fix things.
OOPs = "object-oriented programming", BUT it's a more restrained and thoughtful complaint than just "objects suck" or "inheritance sucks". He cabins it pretty clearly at 11:00 minutes in: "compile-time hierarchy of encapsulation that matches the domain model was a mistake"