These instructions are for a raster image, not a vector. The PDF you posted in the other thread is a 100% vector. To use this method, you’ll need to scale the image to the desired size, rasterize the image (Object > Rasterize > 600 ppi) and then you can tile it across the boards.
If you want to do it with a vector, you’ll need to use a process for splitting a vector, which can be a lot more work.