Missing Code - Different DNL definitions
You are right, in order for a code to be completely missing the DNL has to be equal to -1.

The IEEE standard is just a little bit stricter, it requires that the bin have a minimum width, such that the DNL is smaller than or equal to -0.9.

So, technically for an DNL between -1 and -0.9 the code is there, but according to the IEEE standard such a performance no longer qualifies as "no missing code" in the sense of the standard. It's just a definition.

IEEE Standard 1057 is a little bit clearer:


  DNL is the difference between a specified code bin width and the average code bin width, divided by the average code bin width. When given as one number without a code bin specification, it is the maximum DNL of the entire range. A code is generally defined to be a missing code if the DNL for
  the code is less than −0.9.