Not at all stupid...
Yes, it definitely means "no tweeters", and not a lot of upper midrange - from the cones(s) or from a tweeter. But I do think it means more than that, and it does have to do with the low end and midrange frequencies as well.
Relative to today's rigs, "old school" certainly means lo-fi: the lows are not that extended (think Bergantino HD series, Epifani, fEARful/fEARless, Barefaced, Acme...and yes, AudioKinesis as well), with the low frequencies tending to "bunch up" in the upper bass region (roughly 100Hz - 200Hz), and lower midrange - yielding a tone that was phat, but not really deep.
Since the drivers were not nearly as high-quality when old school was new school, you could expect a fair bit of distortion as well - harmonic as well as otherwise - along with the occasional burp. In general, one could describe the sound as warm, smooth, rounded, relatively phat, but also a bit smeared and blurry. Cutting-edge precision in bass tone was not much of a design goal during the Sixties and early Seventies - nor much of a feasible possibility.
My take on it, at any rate...
MM