They're not holes (though there are probably many vias there to connect to the ground planes and signal traces, but on this board the vias will be tiny and will

They're not holes (though there are probably many vias there to connect to the ground planes and signal traces, but on this board the vias will be tiny and will either be under the solder resist or under the solder blobs). They're soldering lands for where the two halves of the mainboard are soldered together into a sandwich via some kind of thicker PCB interposer, which can carry the signals between the boards. This is probably an order of magnitude or more cheaper and far, far less volume-consuming and way more robust than standard mezzanine connectors. But rather tricky to repair! Look at the video in the article around 7:29. The article calls out the soldered-on heatsink plate, but it's not very clear from the pictures how it actually works in the stack-up without reference to the video. I don't know why they didn't do a better close up of both sides of that, maybe it's in a different article.