I'm assuming they are just spinning up new Android VMs to install the app on so it looks like a first time install and eligible for some free nuggets promotional.
This is exactly how it works, although they've started trying to give you different deals ($1 instead of free, only works on second purchase, fries instead of nuggets, etc)
The app will ban both your IP and MAC address if you use it too many times, so make sure you can either create a new one (via a VM) or randomize it. (Via a custom ROM with MAC randomization enabled)
You don't actually need VMs, you just need alternate accounts on the app. The McDs app is stupid in the way all apps used to be and it doesn't check for unique device IDs or anything before letting you redeem the same offer on a different account w/ same device.