How can one argue it's a monopoly when it's only 25% market share?

multiple aspects:

a) somewhat incorrectly using "monopoly" when they mean "market dominating", which can also apply in non-monopoly situations (and also be regulated)

b) geographical market definitions: countries don't care about global market shares (i.e. your stats), but local ones. Which in the US is 50-60% for Apple. And e.g. in the EU a lot lower, which is why the EU tends to care more about Google+Android

c) market definitions: the app store has a higher percentage of the "mobile apps" market than Apple devices have in the "smartphone hardware" market

Not a monopoly on the mobile market, or even the general app market. They do have a monopoly on the "Apple App Market", which they also abuse to be quite frank.

"Is it a monopoly?" is the wrong question. The right question is "Do they have market power that they abuse?"