Any particular reason why you'd use a Hilbert curve instead of, say, a Z curve (https://en.wikipedia.org/wiki/Z-order_curve)? Conversion to and from actual coordinates seems much more straightforward for that one (just bit [de]interleaving).

I mean, any Quad/Octree/N-dimensional equivalent can have its cells numbered by giving each quadrant/octant/each of the 2^N sub-cells a certain bit combination and then chaining those together as you descend the tree. The Hilbert curve version is just a special case of this with complicated rules for the "sub cell" <-> "bit sequence" mapping. If you were to use a Z curve, the resulting data format and querying algorithm would be exactly identical to the one in the article, just a lot less complicated in the (not presented) details of "where is this child" than the Hilbert version...