Esstee has already mentioned the limitations on using resistors and how it lowers the voltage to the electrodes due to the voltage drop across the resistors/potentiometers.
With regard to the schematic, you could not of looked at it because I went straight to the pdf schematic supplied in the download. It's in the hardware folder. There is a list of components and where to get them, instructions and all that is needed to put it together. There are extensive documents, board layouts, guides on producing the circuit board, source code, guides on putting it together and how to setup the magnetic stirrer.
My view on Steve is the same as shared by Dquixote1217. His information is unreliable an inaccurate and self serving. I do not agree with anything he says. He is your chosen so called expert. There are people on this forum who disagree with that choice, therefore using the term "our" is as accurate and reliable as Steve's advice IMO.