We recommend using SSD for persistent clusters which will make what they call "Logical" fragmentation a non-issue: on SSD reading any block is roughly equally fast.
I guess that "Internal" fragmentation is still a thing. You should expect that Index pages are on average half full, and this is expected. However, in most of use cases indexes do not dominate memory usage so that's not that much important. And for data pages we try to avoid segmentation.