Why can't it be? This is a matter of definition and whether one is comfortable or not with such a definition, which is determined by one's intuitive feel. Certainly, the papers authors are comfortable with it, otherwise they wouldn't use it.
To me, perhaps because of my affinity for theoretical mathematics and also computer programming, I see no problem at all with a crystal as having only one repeating unit. I tend to think of such a thing as being most similar to a mathematical set (though perhaps given the identical nature of the repeating units, a "multiset" might be the better choice if one wants to get strict), and a set can have one or even zero elements within it. That said, a "crystal with zero repeating units" is perhaps uninteresting physically, but mathematically even still makes sense, and moreover, a crystal can be abstracted into its set of lattice points which, as just that, a set, is entirely mathematically reasonable to consider as having 1 or even 0 points. In fact, from such a standpoint, the definition that admits 0 points can be "simpler" when one looks at how it is formulated in formal logic.
Moreover, two atoms constitute only a minimal repeating unit where and when the two atoms are different. If they are the same, then you have actually do have two repeating units in the "lattice".