Apple is known for rejecting apps that mimic the functionality of the website (even the mobile version)
"We should make an app because if we don't Apple might reject the app we didn't make"?
The lead developer is probably more comfortable using Obj-C than web technologies
That is a terrible, awful, no good reason to restrict your target market. If a web site is worth making then it's worth making. Any lead developer worth giving even 1% of equity should be able to adapt to that. If you can do Objective C you can do the web.
The business is designed specifically for mobile users
If it's iOS only then the business is designed for a specific subset of mobile users. That's fine, but let's be clear. A responsive web site is also 'designed for mobile users'.
Eh, it doesn't seem to me that "people in the San Francisco area with access to an iOS device" is much of a market restriction.
Also, as you said, this is not either/or. If the lead was most comfortable with iOS but could also make a web app with a little more doing, then why not launch on iOS and then make a web app once the extra doing is taken care of?
-Apple is known for rejecting apps that mimic the functionality of the website (even the mobile version)
-The lead developer is probably more comfortable using Obj-C than web technologies
-The business is designed specifically for mobile users. Services like Luna already exist; they're trying to tap into a different demographic
-As such, iOS development is cheaper than both iOS and Android