Now that I think about it, I recall you arguing that Jews
weren't actually a nation prior to the Zionist movement. But we've referred to ourselves as a nation for thousands of years. Under your definition, this absolutely makes it true.
Well, no, you haven't. In the first place, you didn't have the conceptual framework to do so until the 18th century, because nobody did. They saw themselves as a "people", sure, but the content and implications of that identity are not necessarily the same as, or even similar to, those of a modern "national" identity.
In the second place, Jewishness-as-nationality was actually a hugely controversial idea among 18th and 19th century Jews, because Jewishness was tied to a religious rather than ethnic community. Jewishness was first and foremost about a particular relationship to god, and while the reproduction of this relationship across the generations was obviously of enormous importance, it could not be stripped of it religious content. In the eyes of Jewish traditionalists, a Jewish convert to Christianity ceased to be a Jew, or, at the very least, they ceased to be a member of the Jewish community. (Most converts would also agree, thinking of themselves simply as being of Jewish ancestry. It wasn't until the 19th century that you get folks like Disraeli, proudly and unapologetically Jewish despite their non-Jewish religious commitments.) The idea that Jewishness was simply heritable, that one could be a Jew simply because ones parents or grandparents were Jews, yet never so much as glance at a temple in one's whole life, was entirely alien to them.
And this isn't simply ancient history, either, because until the post-war period, most Orthodox Jews were deeply sceptical of the idea of a "Jewish nation", equivalent to a "German nation", "Polish nation" or "Russian nation", because they felt it to represent an entirely confused notion of what it was to be a Jew. Mizrahi Jews, similarly, don't seemed to have entertained any particular concept of Jewish nationhood, in part simply because "nationality" wasn't a category that figured into the self-identity of most people in the Middle East.
On top of all this, I'll also say that "truth" simply isn't an appropriate category. Nations are works of collective-imagination, so when talking about nations, we're talking about what and how people imagine themselves (and to some extent how they imagine others). It is not that people bring a nation into being by imagining it, and that it ceases to exist when they stop, but that it only ever existed as an ongoing act of imagination in the first place.
I used to think the same way, yet if you could graph the social ties of all people of several countries, that could probably end up in a rather neat correlation with nation states - mar political conflicts within states - that cannot be explained by geographical and political limitations. And I don't see why it would be incompatible with the notion of measurably existing nation states and the idea that nations change.
You wouldn't actually be measuring nations, though. You'd be measuring certain kinds of social activity which are taken to express shared national identity. And it's by no means obvious that it doesn't work the other way around, that repeated interaction generates shared identity. (It's most likely seem measure of each, and that measure is probably another variable.) So national identity remains an act of imagination, and while your proposition might give us some insight into how nations are imagined and how that imagination changes over time, it doesn't actually affirm nations as actually-existing entities.