It is on the face of it a pretty weird phrase ("subject matter"), but I've always seen it used to mean what mindcrime means. The phrase is also used in general

It is on the face of it a pretty weird phrase ("subject matter"), but I've always seen it used to mean what mindcrime means. The phrase is also used in general non-ML specific programming (eg among a team of people building a mobile app for a bank, the "subject matter expert" might be a non-programmer who has years of experience working for banks). It's also disambiguated by the author saying "subject matter expert for your application" (application being an application of machine learning, eg to radiology, geology...), and saying "subject matter experts in their application but deep learning novices" in the paper's abstract.