the .brd file is just for the silkscreen, I guess it’s to help you locate components when you are building the kit.
The front and back pdf do show the tracks though, so technically you could use that to redraw the tracks between the components on the .brd file for a join-the-dots from hell.
Yeah, that’s a little strange. brd is Eagle PCB design file format, importable into KiCad, but as you say it only shows the silkscreen. Well, silkscreen, pads, and fab layers, but no copper. They give you the schematic as a PDF, not anything you can work in Eagle or KiCad with, they give you a PDF of the PCB layout with all the traces, but they withhold the complete circuit and PCB design files. So it’s sort of almost open source, but not really. Doing your own board would be in principle possible but in practice a massive pain.