Apple had the first smartphone. That means they had 100℅ of that market.
Of course they were not first. Smartphones were already selling at over 100 million a year by the time Apple got involved.
In fact, the primary reason why Apple scrambled to put out a smartphone at the time, was because they saw how quickly the smartphone market was growing. They correctly understood that their iPod market was soon going to be really threatened by all those combo phones+music players.
Btw, the debut of the iPhone barely affected the overall smartphone sales trajectory. What DID cause a huge burst in sales, was when affordable and available Android phones finally hit the market:
Of course, that likely wouldn't have happened quite the same way, without Apple making more people aware of the desirability of owning a smartphone.