Definition of "essentially diploid"
I think that the rest of the paragraph that you left out of your quotation helps to clarify. The CHO have 21 chromosomes, while the somatic lung cells have 23. 

Neither case is exact for Chinese Hamsters, 2n = 22. So they have almost the complement of chromosomes that a diploid Chinese hamster somatic cell would have but they are missing one or have one extra. For the most part they will behave like normal somatic cells in culture, but it is an important fact to note as you can have cases where dosage dependence of a missing chromosome in a pair or an additional chromosome (trisomy) can affect the outcomes of research.

In the back of your mind you always need to be thinking how does the missing/extra chromosome affect the data I am collecting.

This Nature article may also be of interest.