I mean, in a way, a pointer is just an array index into heap space? Such that, I think this is accurate? I'd drop the "just", though. They can be thought of that way, but the idea is how to keep the programmer from having to do that manually.

To rephrase, this is how to mechanically bit-pack indexes to save space. Doing it in a pointer means you are likely wanting to share these pointers with other parts of the code, so you also need to have them be self contained. Naive way would be for each pointer to be a tuple identifying arena and having its offset. But there is still a lot of effort that has to be done for that to work well. This paper looks at that effort and gives a pass at how worthwhile it is.