Nope, no Arduino involved! The original Form 1 and 1+ used an STM32F103. Some of the firmware used libmaple [1], which exposes an Arduino-ish API, but most of the code was low-level C and C++.

(I'm an electrical engineer at Formlabs, and was around through the Form 1+ product cycle)

[1] https://github.com/leaflabs/libmaple