Don't have it to look at, but yeah,
prior to Tango becoming an "accepted" dance, it was largely without form or structure, being made up by the dancers
and most of those dancers were blacks and lower class whites, and the dance was smoothed out in Europe and then it came back to Argentina.
I happen to be reading "Swingin' the Dream" and, as you write, initially people "make things up" which means they combined elements from other "dances", and/or drew from "venacular" movements, etc.
But, in the history of both dances, people began teaching, not peer to peer as part of an informal transmission of knowledge, or learning by watching, but as teachers with students and with money (or maybe other services) changing hands. For tango that happened in Europe. That's when all the "racey" stuff was taken out. That, I think is where the improv part was lost.
With "swing" even the Savoy dancers, the best of whom were professionals, sold lessons to others (although in general the dance spread informally without a host of teachers). "Swing" as a dance wasn't used until after the term "jitterbug" had run its course. And jitterbug was a catch all that included Shag, Suzi-Q, Truckin', etc. Far as I can tell the term "Lindy Hop" was localized around NYC. (in LA it was called the New Yorker)
In reading about how people who lived outside of cities in the US danced, there was a lot of variation, to my way of thinking, improvising. This has a very low profile in dance history.
I bring this up because waltz, one of the most "ballroom" of dances, was taken from a dance or dances that were danced in the countryside. No doubt it became codified in the courts and ballrooms.
So, I see a lot of similarities in the history of these dances. And they all seem to start informally and with improvisation, which is then lost when it begins to be "taught" by teachers who have students.
So maybe a more precise quesiton would be, when did AT again start having an emphasis of being improvised, and is there any documentation of that.
Maybe it's always been that way, but then the quesiton is, why wasn't it "written up" that way? Or was it?