If you do a bit of Googling you can get some idea, there’s software out there which can emulate parts of the Machinedrum which I believe they used to help them figure out how to reverse engineer the firmware.
But it must have required a huge amount of patience, perseverance and talent to gain a full enough understanding of the (un-commented) assembly code in the firmware image to make these changes, it’s an incredible achievement and even if the methods to extract machine code from the firmware were public, it would require so much work to get to the stage that this is at.
It’s a bit of a shame that Elektron aren’t OK for them to open source this project, I personally would love to take a look at it, more out of curiosity than anything… but at the same time there is something to be said for a small team who understand the project intimately, and it’s cool that it has any kind of blessing from Elektron