What he meant was: if Windows 8 sells like Windows 7, and it will, there will be a mind boggling number of users to target. Windows 8 market will quickly be bigger than Mac OS X market because Windows 8 will ship on every new PC being sold.
And since there are no apps, there are lots of opportunities just writing the same kind of apps that people have written in the past, except for Metro. It'll be a new gold rush and you can trust me, developers will rush for that new gold.
Well, I don't think anyone's expecting it to sell like Windows 7. They're expecting it to 'flop' like Vista. i.e. sell quite a lot but have a large user base sit on Windows 7 waiting for Windows 9 as people sat on XP after Vista.
And since there are no apps, there are lots of opportunities just writing the same kind of apps that people have written in the past, except for Metro. It'll be a new gold rush and you can trust me, developers will rush for that new gold.