Even the clones give credit.

I don't see any mentioned of Plaits or MI. Given that the work is derivative of MI Plaits they should acknowledge as such and give credit to Emilie Gillet. The licensing on the GitHub page for MI (https://github.com/pichenettes/eurorack) says it is GPL3.0 in the case of Code (AVR projects) and the MIT license in the case of Code (STM32F projects). My reading of this is that in both cases, the license and copyright has to be published with the derivative work and in the case of the GPL 3.0 the source code of the derived work has to be open-source and provided with it.