Bitkey replied on my Twitter. It's a "secure MCU". 

This is from https://www.ledger.com/ledger-101%25e2%2580%258a-%25e2%2580%258apart-2-why-hardware-wallets-are-secure

There are two types of microcontrollers:

the “normal” one used in your microwave, hobbyist drone or TV remote controller. It’s called a MicroController Unit or MCU in short.
the “secure” one used in payment cards, SIM cards or passports. It’s called a Secure Element or SE.

So it's just a microcomponentized computer.