[_delirium]

It wouldn't be hard to define up front, but in context I don't think assuming that the audience knows what MPI is poses much of a problem for the goals of the piece. This isn't a general-interest piece about HPC, but specifically an advocacy piece attempting to convince members of the HPC community that their strong attachment to MPI is detrimental to the field, and that they should refocus their efforts elsewhere. If someone doesn't know what MPI is, they are probably not strongly attached to it (and probably not in HPC), so aren't the people the author is trying to convince.