It’s not an anti-trust if they would simply drop Qualcomm modems and use their in-house modems for their own devices only. Prior to the iPhone 4 with the in-house A4 chip, iPhones 3GS (600 MHz) and older uses Samsung designed SoCs and yet no one cared.
But it does become an anti-trust issue if iPhones take the majority of the market; leaving the rest with only a fraction of market does become anti-trust.