Domen
Misico dux Vandalorum
Whatever meaning they meant (probably to show most of their population is of German region)
They meant exactly the same - or a very similar thing - as we mean today, and our ancestors meant in 1900, when talking about nations or ethnic groups. I'd like to notice that since the 19th century until today, there is no consensus when it comes to definition of nation or ethnic group. There exist many, more or less different, definitions. But there is no reason to assume that before 1800 the variety of different understandings of these concepts was not the same as today.
HRE certainly was not nation-state
You and me know this, but majority of deputies who gathered in Cologne in 1512 - and majority were Germans - didn't know this.
Probably they were influenced by Martin Luther or someone like him, who told them that Slavic-speaking Sorbs (Wends), as well as Jews, etc., were "Roman" citizens of 2nd category, and therefore the HRE should have been renamed to the HRE of the German Nation. This is what Martin Luther really wrote.
BTW - Mozart described himself as German. And Voltaire mocked Germans that they were calling their country "Roman", despite being Germans. Both guys lived long before 1800, and they were well aware of what nations and ethnic groups are, so I'm not sure why this modern post-1945 myth (yes - before the horrible things of WW2 took place, for which nationalism was blamed, nobody though like this) that nations were "invented" in year 1800 is still so popular.
Do people fear that if we finally admit that nations are much older than 1800, immediately WW3 will start, or something like this ??? Silly fears.
BTW - the "invention" of nationalism is sometimes attributed to the French Revolution (ughr!!! these evil and arrogant French "frogs" again!!! they caused WW2 by "inventing" nationalism, not peaceful Germans!!!

But attributing the "invention" of nations to French revolutionists is wrong, because they are much older, and were not "invented" but aroused.
So stop blaming the French for all things bad, dear Tea-Drinkers, or dear Cabbage-Eaters who invented the nation-state of Cabbage-Eaters already in 1512.