He's talking about mobile platforms. Microsoft is very much the underdog after ignoring the platform for so long. Apple had a monopoly over good smartphones by beating everyone to market by such a large margin, though it's hard to call them a monopoly now that Android has been selling so well.
In fact, if I walk into a mobile phone shop, I've overwhelmed with competing products and can hardly see the iPhone.
Five years ago you could have made an argument for Symbian being the Evil Monopoly in this market, but Apple/iOS has never been anything like a monopoly. People seem to confuse control over one device with an actual monopoly. They're not even remotely the same thing.
They have 90% of the mp3 market and 75% of the mp3 player market but that's still not a monopoly.
The only thing they could be accused of is a monopoly on non-crappy web apps. Pretty amusing.
(As of Q1, 2010, pre-iPhone 4, source: http://www.gartner.com/it/page.jsp?id=1372013)
No one seems to mind their MP3 player monopoly because the iPod was so much better than the alternatives.
Smartphones aren't expected to reach 50% mobile market share until Q3 2011.
Most people on HN have a really poor understanding of the global market. The US leaders are not the world leaders in the mobile space.
Example, iTunes is nearing monopoly status in that they can largely tell the industry, "I don't like your DRM proposal, and we're so big that even if every other company wants to do it one way, you can't. You have to listen to us".
Now remember monopolies are not illegal. But actions of monopolists can be.
Your point stands, though.