Those theorists didn't originate the relatedeness, they explain how this relatedeness was originated and the history of how that word as changed to justify a certain sentiment or notion.
The very term natio, as was used three thousand years ago, had a different meaning than it had three hundred years ago.
During the classical Roman period, the term natio was used to refer to a specific lineage of a family group, dynasty, or Patrician rank. Fastforward to the early modern, and modern periods, the term came up to signifying entirely vast populations, with no actual direct lineage or familial relationship - or even without its members to actually know each other - but within the same identity as we now define what a national group is.
Therefore, yes, the term has changed its meaning considerably over the years, and that change was shaped by the historical process. Nothing of it is "natural" because historical processes, by definition, are not "natural" events, but the result of rational human decisions. If we disagree with that very basic premise, than we disagree with basically everything, which makes further conversation between us futile, and quite frankly, boring.