The article, which is worded strangely at some points, seems to take a generally anti-nationalist position. Thus, I will try to defend nationalism.
The fact that humans are social animals is well known. A social animal is that which requires the company of others in order to lead a healthy and full life, both physically and psychologically. As individuals, we crave to be part of a larger unit, a social system that is made up of related persons who are more than their sum. While it is easy to form an informal group, a social system (i.e. a society) requires a series of factors to be in place, the most important of which being cooperation. Cooperation itself also has requirements, the most critical of which that I can think of right now being communication and trust, these two also being related to one another, as I will explain further ahead.
Communication takes place when there is transmission of information through some sort of code that both entities can comprehend. In the case of individuals, this means a language. Also in the case of people, there must be a measure of trust that the information being transmitted is truthful, otherwise it will be immediately dismissed. The action of communicating something not only has a very significant practical relevance (because of cooperation), but also holds a symbolic importance in the idea of sharing with another a piece of your thoughts, and thus yourself, and believing that the listener/reader/spectator will comprehend you.
Trust is a belief that something is reliable, dependable and certain. To trust someone is to hold the belief that one can depend on that person, either in a specific situation (like a patient trusting a doctor to operate on them) or in a general one. Without trust there can be no cooperation, because every individual in the group would need to constantly verify the results of the actions of the others in order to believe that they have performed them, and done so correctly. A group without trust is worse for the individuals than being by themselves, because all of the people around them are not only almost useless, but potential threats. Communication plays a role in trust because it allows individuals to present themselves, argue why they are worthy of the other's trust, and create a bond in the form of contracts and informal relations.
How does this relate to nationalism?
Both of the critical requirements for cooperation can more easily come to be with the presence of similarities between the individuals. Individuals who speak the same language already overcome the greatest obstacle in communication, individuals from the same culture will better understand the particularities of the communication, and the more similar to the speaker the listener is, the more likely they are to fully grasp what is being transmitted. Trust is also benefited by similarities in obvious ways; since most people think of themselves as trustworthy, the more similar to themselves the others are, the more likely they are to be trustworthy.
Cultural similarity is the most important kind of similarity for societies, since it has the most affect on behavior. Considering that every society has its own culture (for culture can be defined as all of the non-physical factors that differentiate societies) and individuals are part of many societies at the same time, a person's cultural background can be described as a very complex three-dimensional Venn diagram, which encompasses social class, religion, family and so on. While most of these can only be evaluated after a bit of dialogue (and even so only if the person chooses to tell you this), language, nationality and ethnicity tend to be the first things that people notice in the physical world. Residence in a similar geographic area means that the individual likely has several common experiences in their past, resulting in a similar personality. Ethnicity is instinctively related to location, and also to an animalistic act of physical comparison. Language is somewhat self-explanatory.
My point boils down to this: Nationalism reinforces the notion of collective identity between the members of the group, thus strengthening the social fabric through increased perceived similarities. While this has positive effects among members of the same nation, it can have negative effects for the outsider who tries to gain entry into it, as they can be seen as an invader, a diluter or a polluter of the population pool. Condemning nationalism is to condemn a behavior rooted in some of the basic instincts of man, while at the same time dismissing and negating its benefits.