Redeem is the "firmware" that runs Replicape. It has been developed with a focus on hackability with only a small code base written in C and optimized for fast execution. That is because only a small part of the code needs to be executed fast. Most of the code only needs to be maintainable and readable. And for that, there is no better language than Python.
Redeem does something else that most 3D-printer firmwares does not do. It precalculates all path segments before shipping them off to a separate Programmable Realtime Unit (PRU) for execution. That leaves the main CPU free to handle path planning and temperature handling using a normal Linux kernel while the stepper execution and end-stop handling is done by two 200MHz dedicated microcontrollers. Compare that to the single 16MHz Atmel uC that runs the vast majority of todays 3D-printers.