Making birch tar at all is a fairly complex process. It takes multiple steps, lots of planning, and detailed knowledge of the materials and the process. So the fact that archaeologists have found a handful of tools hafted using birch tar tells us that Neanderthals were (pardon the pun) pretty sharp.
But the Zandmotor Beach flake tells us more than that. Making birch tar adhesive for tools was so routine that Neanderthals would do it even for a simple domestic tool like a small flake—even in the extreme environment of Ice Age Northwestern Europe, in the shadow of glaciers at the very northern edge of where Neanderthals could survive. And all the while, they were using fairly advanced methods for more efficient production.