Hey, good catch, the answer key is a bit weird in its wording.
Those two molecules are diastereomers, but the answer key words the explanation weirdly.
Note that they say that the two molecules are not non-superimposeable mirror images, implying that these are not enantiomers.
I would rephrase this explanation to say that these molecules are not mirror images of each other, making them diastereomers. However, they are certainly still non-superimposeable upon each other.
You are correct in saying diastereomers are non superimposeable molecules that are not mirror images, and that book agrees with you, it's just a somewhat clumsy wording.
Please let me know if this is still unclear.