That's not how any of this works. Majority share is a lot of power, but the theory of how monopolies work doesn't support the idea that anyone with a majority of the market has monopoly power.
It's most definitely how this works. Monopoly power isn't some binary attribute. It's not like you flip a switch one day from not being a monopoly to suddenly becoming a monopoly. Monopoly power is accrued over time with increasing market share. Very few companies have the kind of market share we are talking about here with Apple and the US mobile OS market. Take for example, Walmart, who many people think of being gigantic in the retail sector in the US. They only account for 16% of US retail sales. If Apple only had 16% of the US mobile OS market there wouldn't even be a discussion about anti-trust laws and Apple wouldn't have just made this announcement -> https://developer.apple.com/news/?id=84w3e5bm
