I'm not exactly sure about the technical details, but that "can't install on the device" notice is what everyone will see until they send us the email that requests access. The email automatically submits an iPhone-specific identifier that is used to install the application. I think this is what is usually required of testflight, but I could be wrong.

We're all on the 4S, so once we get the email we'll do our best to add you to the testflight list as soon as possible. Sorry for the confusion.

And we've heard from a lot of Android users, and we feel your pain! One of the developers on our project doesn't even own an iPhone so you have someone vouching for you on the inside. It's definitely on our list of to-dos.