I think it's all about how you define the market. Is the market for "apps on the iPhone" or "computer software in general"? Apple has monopoly control of the former but not the latter.
(I personally don't agree with this viewpoint but it's how it is viewed by many. For one, I've spent $X on iOS apps, if I switch platforms I lose all of those apps. Feels unfair!)
Reality is anyone releasing a for-profit app aiming for mass appeal can't afford to ignore either the Android or iOS ecosystem. They have to release for both, so there's no real competition involved.
Apple has a monopoly on its app-store the same way Amazon has a monopoly on what (and how) it sells on AWS for services.
Even if one builds for the web, apple only allows a single browser engine which is their own. They unilaterally can decide what to implement and what features to add or remove.