We've submitted feedback to Apple on this confusing part of the App Store. The "Compatibility" section that lists "This is app is optimized for iPhone 5, iPhone 6, and iPhone 6 Plus." refers to the screen resolutions (sizes) of these devices. i.e. iPhone 5 has a 4" diagonal screen, iPhone 6 has a 4.7" diagonal screen, and iPhone 6 Plus has a 5.5" diagonal screen.
However, in order for Lark's activity tracking, to automatically log walks/runs/bike rides/etc., your iPhone must have the built-in low-power sensors (M7/M8), which exist in the iPhone 5s, iPhone 6, and iPhone 6 Plus.
We're working on expanding functionality, for example if you have an Apple Watch and an iPhone 5, or if you use an app like Human. Sit tight as we work out the details, and thanks for being patient!
You can see the "Compatibility" section in the image below.