BUT ocaml also has row polymorphism and polymorphic invariants, which you can use to get something much directly closer to c#/java interfaces. The major difference is that with ocaml it's structural typing and with c#/java it's nominal typing. However, at a high level you can write the code in a very similar structural pattern as with what you get in c#/java.
Clearly not considering he mentions typeclass in the article.
Module signatures are similar to interface and parametrized modules allow you to do what you would do with abstract class: you define a signature and then write a module using these functions. Parametrized modules are more or less a generalization of functions which goes from module to module.