You're mixing the literal definition of monopoly with anti-trust laws. They have over half the market as a single company and the rest of the market is actually a fragmented zone of other companies so yes I think they are. You don't have to own the entire market to run afoul of monopoly laws they don't require there to be literally only one choice in the market.
Not a lawyer (let alone one specializing in antitrust law), but it looks like the relevant legal standard is "dominant position". Basically, it's legal to have a dominant position, but that position can be abused through certain categories of actions. By contrast, under the Sherman Act it's nominally a felony to even attempt to become a monopoly (although the application of this by courts has apparently been both complex and contentious).
