After struggling for a bit with the HLOD system, which is absolutely magnificent btw; I realized it only takes in non-movable geometry. Which as obvious as it might seem in hindsight should totally be mentioned at lest once somewhere in the documentation.
Inside the editor it does show a tool-tip about mobility, but only if you try to add geometry through a drag-n-drop operation from the world outline to the HLOD outline. Otherwise it just doesn’t work and doesn’t tell you why anywhere. ( I had to look in code to see what was going on )
This is important, I think, because when you’re working with blueprints, they all have everything set to movable by default D: