The Arduino Uno-compatible board has an MCS-51 (often called 8051 instead) instead of the usual ATmega328P/ATmega168. Specifically, [ElectroBoy] uses the AT89S52.

The title of the smallest Arduino board doesn't go to just a single board — it actually belongs to an entire family: the Nano family. This set of boards comes in the smallest form factor Arduino ...

The Arduino Nano and Uno are equipped with very similar processors (the chip that essentially serves as the brain of the board). The Nano features an ATmega328, while the Uno sports an ATmega328P.