The ad copy is saying that the frame has some of Tange's 531-esque product of the time, i.e. a steel with manganese and molybdenum alloying elements. It's either all Tange, or some Tange and some 1024/hi-ten tubes, or Tange frame/1024 fork, or some mix thereof. None of that would be unusual. The proud "1024" logo is there to play to the notion of the buying public being impressed by assurances that the frame is made of something nicer than straight melted Chevies, where some care and QC went into producing the material for its tubes. High-tensile steels do in fact have good properties for making bikes compared to cheaper alternatives, which bikes were also once made of. Today it's true that something like 4130, which is much better yet, is rightfully viewed as more of a baseline for a "real" bike, but it and similar nice-but-not-extravagant steels didn't have that position in the 70s.
A rule of talking about comparing steels as they're used in bike frames in different historical eras is other than for some very specific intents and purposes, there's not a conversation unless you're also talking about wall thickness. A question you could ask is whether the transition from the lug to the TIG era brought with it any changes in wall thicknesses typically used along with 1020-ish steels, since TIG processes are more sensitive to distortion and will run faster and cheaper if thicknesses are increased, even if it's unnecessary on a strength level and takes away from light weight and ride quality. Some contemporary steel bikes suffer greatly from this, including 1020-ish ones and even many basic chromoly (4130, Reynolds 520, etc) ones.
Midrange and nicer Japanese bikes from this era are usually very well-made regardless of the material, and so it is true that 1020 type materials on a bike like the one in question were likely handled with more respect than usually found globally today. (This is not to say there's no variance today, and you'd also likely be comparing a mostly TIG versus a mostly brazed bike so some elements become apples-to-oranges). It is critical though to point out that that's more characteristic of Japanese bikes above a certain quality tier than it is of bike-boom era high-ten bikes in general. As a whole, many 70s and 80s hi-ten bikes had terrible production quality (gappy brazing, rough lug finishing, etc) that barely passed in use.