"we have a particular use-case where we have a large contiguous array with, say, 100,000 lines and 1000 columns"

This is where they lost me. This is NOT a lot of data. Should we be surprised that memory-mapping works well here?

Below about 100-200 GB you can do everything in memory. You simply don't need fancy file-systems. These systems are for actual big data sets where you have several terabytes to several petabytes.

Don't try to use a chainsaw to cut a piece of paper and then complain that scissors work better. Of course they do...