Much like a ‘cocktail’ is sometimes anything served in a cocktail glass, a ‘highball’ is sometimes anything served in a tall glass.
I think one of the earliest important cocktail books to feature Highballs is Charles Mahoney’s 1905 Hoffman House Bartender’s Guide, and some of those are pretty elaborate:
The template of ‘spirits plus a large amount of mixer’ is lurking in this one, but the spirits portion has been chopped up into a cocktail of its own!