How do Apple's USB Power Adapters allow for more power output than is possible in the USB 2.0 specification?
In the official website, it say "USB Power Adapter" without specifying which version of the USB port, why do you think it's 2.0 version ? 

Also, the power allowance from USB 2.0 specification is for powering connected devices (mice, keyboards...). For charging, there is Battery Charging Specification

You can read more in here:
https://www.maximintegrated.com/en/design/technical-documents/tutorials/4/4803.html