man... that is about the hardest way in the world to print a job
Back in the 1970's I wrote a ton of code to do exactly that with pen plotters. There was no such thing as a color printer. Hell, there weren't even any color monitors. If you wanted color output, you used a pen plotter.
Doing an object fill is simple, just a bit of simple algebra calculating vector intercepts and knowing the pen tip diameter.
With an 8 pen plotter, that's all the colors there were, we even did what passes for photo reproduction with some primitive dithering algorithms.
Monochromatic photo output was even simpler than filling objects, merely resolve the bitmap to match the pen diameter and then either pen up or pen down for each pixel. Contiguous similar pixels in the direction you were scanning the bitmap were rendered as a line, singleton pixels as a dot. You could scan the bitmap either horizontally, vertically, or diagonally depending on what you felt like doing.
It was slow and, looking back, it was silly, but back then it was the only game in town. Sort of like during the California gold rush in 1849 dirty laundry was shipped to Hawaii and shipped back clean. Or the Pony Express. You had to start somewhere with the tools you had at the time.
I wonder what software they are using and how to make Flexi do the Fill In as opposed to just the cut lines.