They are a natural monopoly. All network-based marketplaces with barriers to entry are.
That competitors exist does not make them not a natural monopoly. Even the fact that the are not a current monopoly does not mean they are not a natural monopoly: https://en.wikipedia.org/wiki/Natural_monopoly
The first 6 worlds in your cite are "a natural monopoly is a monopoly...". That competitors exist mean they're not a monopoly and therefore can't be a natural monopoly.
>Specifically, an industry is a natural monopoly if the total cost of one firm, producing the total output, is lower than the total cost of two or more firms producing the entire production.
The industry of operating a network connecting independent taxi service providers to taxi service clients is the natural monopoly, simple because the cost to operate the service is wildly scalable, and become more useful as more people use them, as all network-based industries are.
