Interesting but it looks to have been filmed a long time ago, can you still buy Belgian stones?
Ardennes markets them, but they are far from cost effective. I didn't watch this video, but understand something like 7 different firms were offering belgian hones at one point, generally taken where the veins meet (blue on one side, yellow on the other). These are referred to as "natural combos" now and expensive.
I've had somewhere around 10 or 15 belgian stones, and the new types are OK, but on average do not measure up to vintage stones.
Separate comment, too - the older stones (and maybe the new) were graded for use - they come in a range of hardness and a range of fineness. If you're looking for a finish stone and you get a coarse one, it won't return an edge that you will like too much, but I'd be willing to bet that there are a lot of these still in use in continental europe as finishing stones.
Not sure if ardennes mines belgian blue stones for architectural use, but it appears that much of the mining was for architectural use (the blue is a pretty stone, uniform and fine and probably easy to cut and manipulate).
So, why are they not cost effective now? Just my opinion, but there is a limited source and a huge market of internet readers, which means when they sell a stone as "select", it's their grading. I've had stones that were slow and not that fine (from ardennes), and a bout that I have now that's fast on slurry, and slow on water, which is a nice combination, but also a bit coarse. A small offcut "bout" #10 size is probably about $125 now. Full size stones are likely in the $300-$400 range (I haven't looked) if you're considering something bench stone sized and on average, they're not that fine, and would need to be used for a while with no slurry with a light touch to get a really fine edge.
They are sort of like arkansas stones, though, in that if you can use a surface on water only with light pressure, you can get an edge finer than the underlying particle size because the particles aren't aggressive like synthetic alumina.
They are also like many natural stones in that a few of them are real bangers and you put a tool to them or a razor and you have a finely honed razor or sharp tool. I think the variability adds to the hunt for a lot of folks. Out of the 10 or 15 that I had, mostly sold through razor avenues, two were actually really fine. when I honed tools on them with a bit heavier hand (something you would do that's practical as you won't be inclined to finish hone a tool for 100 strokes with light pressure), the edge wasn't as good as more common stones like a charn or fine arkansas stone.