I do not take it that Comcast is charity, so either they made a horrible deal and lost significant amount of investment, or the narrative is lacking some details. I personally doubt we will find a $250 million investment in Comcasts books regarding the city of Baltimore however, as I find there is simpler narratives that would match in how cities like Baltimore gets fiber into peoples homes. It would also explain why some cities has has semi-monopolies and others don't.
A city that wants fiber has the obvious choice to subsidize the investment of a large ISP, say Comcast, thus severely reducing the investment cost. The permission to build is of course non-exclusive, but the subsidizing is not. Once built, the ISP gained a government granted advantage, and thus a monopoly is born.
Alternative, a city can create a government bid for administrating and building a city fiber network. On paper, this mean that the city owns the fiber, and the ISP that builds it has to sell access on equal ground with competitors. The ISP do still get an advantage in operation knowledge, some payment for doing the administration, and first mover advantage. This tend to result in healthy, if somewhat constrained, competition.
Ars Technica narrative talk about how the cable division paid little or no construction costs. That is very different aspect than if the permission to construct, operate, repair, maintain, and reconstruct a Cable System is non-exclusive.