He didn't say that MSFT and Samsung weren't competitors of Apple. He said MacOS and iOS are monopolies. Those are two different things.
You can switch from a monopoly over a specific service/product to a similar but not identical one. MacOS has applications and features that Windows doesn't. It historically enabled features (sealed batteries) that Windows didn't, that provided better performance.
And MacOS can run linux and windows. For people who value those things, MacOS is a complete monopoly.
iOS is even more differentiated from Android. Android won't support all the music/movies/shows I've bought from iTunes, does't work with iCloud, etc. It doesn't airplay to my other Apple devices.
These "monopolies" are why Apple is successful. When they switched to Intel they didn't lower costs by adopting commoditized PC hardware designs and outsource OS development to MSFT by licensing windows. They used Intel to provide more powerful and more flexible Macs, and continued to differentiate themselves by adding value with aluminum bodies, retina screens, mag-safe adapters, thunderbolt, etc, etc, etc.
I think Apple understands much better than most of us geeks that their customers spend a lot of time with their devices and on their Macs. If I pay $1000 more for an "equivalent" Mac than Windows PC, it costs me about 50 cents more per work hour. Which means my payback needs less than a 1% increase in productive. And it's trivial to get far more than that because despite having similar processors/ram/storage, they are in no way equivalent in build quality, flexibility, and usability.
Where Apple's model fails is for the people who don't make much per hour, so building your own, or getting the cheapest, makes sense.