I would have thought that the map matches density and growth because the streetcars themselves were the main agents of urban growth in the late 19th and early 20th centuries.
Many streetcar companies had close ties to real estate and land development companies. Land is essentially useless for development unless people can get to it, and go to work, shop, etc. Streetcar lines allowed real estate to development. Thomas Lowry, who can be considered the principal founder of the first horsecar lines, as well as the electrified TCRT, had extensive dealings in land and finances.

A few key quotes from Russell Olson's "The Electric Railways of Minnesota".

"He (Thomas Lowry) believed in constructing in advance of the population and waiting for traffic to slowly follow."

"The extent of urban development was limited almost entirely by the operating radius of the street railway. The electric railway, relatively inexpensive to construct and operate, and faster than either the horsecar or cable car, changed the character of the city. Streetcar lines established the pattern for growth within the city."

In short, I guess I'm not exactly sure why this is considered surprising. Take a look at a TCRT map from the 1950's, then look at a modern Metro Transit system map. They are remarkably similar, particularly Minneapolis and St. Paul proper. The buses do the same routes that the streetcars did, because that is where the population patterns still are. And the population patterns stayed there because the bus lines perpetuated the legacy of the streetcars.

As far as density, streetcar lines tended to foster a low to medium density growth. Think of South Minneapolis, that kind of density. Because the streetcars were cheap, and land was cheap, residences did not have to be packed in as dense as you would see in older time periods or in the urban core. Just like freeways and cars easily extended the commuter distance up to 50 miles from an urban center, streetcar lines did the same thing, only to a lesser scale. The land development companies wanted to sell land and homes, so they would do everything they could to discourage density. This lack of density in many areas later came back to hurt the streetcar lines, because there were not enough riders to justify the high fixed operating costs.