Apple did not have a monopoly on podcasts, they were basically the default option for consumption on iOS and a LOT of people have iOS devices. Anyone can make a competing podcast app - that is not to undermine the difficulty of getting any significant market share here - but the barriers are not that high.
True monopolies have built in market barriers that is impossible without nearly unlimited monetary and political capital.
An example of a true monopoly might be your local electric utility company. They have a monopoly on delivering electricity to your home and (in certain areas) a monopoly on generation as well.
Even if you had the political backing - the enormous amount capex to acquire any amount of marketshare just to deliver a commodity with relatively slim margins means no startup could ever tackle it.