Don't forget that the iPhone was only available in a handful of countries in 2007... US, UK, France, Germany, Portugal, the Republic of Ireland, and Austria. And it was launched in June... halfway through 2007.
The iPhone eventually became available in more countries in later years.
Hell... it took almost 4 years for the iPhone to be on more carriers in the US... remember AT&T had exclusivity until 2011. So even in its home country there were tons of people who didn't want to switch carriers to get the iPhone.
So yeah... maybe the App Store was the reason for the explosion of sales... but I think it had more to do with general availability and people's acceptance of a new style of phone (switching from flip phones, Blackberries, etc)