I don't think that Android would have had better apps if Android developer tools were Mac-only. The reason behind the Play Store/App Store quality difference has more to do with the way the store is run, the system frameworks (Core Animation vs. the Java frameworks, etc.), and the inclination of users to purchase apps.
A broader base of developers can mean more and higher-quality apps. A better selection of apps can lead to greater sales of iPhones and iPads.
(It also might not. But there is certainly a potential benefit for Apple here.)