Ah right, youre right it's probably the interleaved bits (aka Morton code) in actuality. Or more likely, something tuned to the specific cache sizes used in the GPU.
Texture memory is still stored like that in modern chips (presuming they meant Hilbert curve organization). It's so that you can access 2D areas of memory but still have them close by in 1D layout to make it work with caching.
Ignoring all the issues with decoding any message after 500 years... Semiconductor based devices will stop working by then due to diffusion. Maybe a large conductive structure that creates EM backscatter/interference that encodes your website? Or perhaps something that attracts lightning strikes and uses them to emit a burst of information.
Also, if your website really does last and remains decodeable that long, and if it is an exceptional occurrence, then it could also become a target for destruction if the winds of culture shift in some way.