Does fileDatastore not implement "matlab.io.Datastore"?

I'm currently developing a custom datastore that takes an existing datastore and adds some features.

In the constructor I would like to make sure that a passed argument is of type "matlab.io.Datastore" so I do:

assert(isa(ds, "matlab.io.Datastore"), ...)

This works fine for another custom datastore that directly implements "matlab.io.Datastore" but it does not work when das is created using

ds = fileDatastore(...)

fileDatastore. Form digging around in the source of "matlab.io.datastore.FileDatastore" I found out that it at some point implements "matlab.io.Datastore" but still above is not wokring. Am I missing somehting obvious?