One of the huge benefits people who dismiss mobile first is the ability to control the entire experience. If you launch an iPhone app, you have pixel perfect accuracy, fast speed and can make sure your app works perfectly on all 5 of the devices out there.
Yup, it does narrow your market even further. But not in a very meaningful way. iPhone owners are drastically more likely to download and use your app than on any other platform.
The hard question is whether to launch on the web or mobile first. Once you've decided on mobile first, choosing iPhone first is a fairly obvious choice.
It's not always about reaching everyone as fast as possible. Going wide initially is usually a first time entrepreneurs mistake. Build something, figure out that your value proposition is compelling for your target users. As soon as you have hit "product market fit" go wide.
There is a reason companies are going iPhone first.
There are many potential narratives that explain why companies are going iPhone first.
An alternate explanation to yours is that companies are going iPhone first because it's trendy. We all know about the SV echo chamber. Following trends seems to fit the data just as well as your hypothesis.
