mpweiher could mean implementation theory, so things like how to write better compiler analysis and optimisations and things like that, but in the academic and research community I think that just isn't thought of as coming under the term 'programming language theory'. It's not thought unimportant, it's just a different topic, more grouped under 'systems'.

Looking at mpweiher's profile it looks like he might be interested in things like metaprogramming and object protocols - again they're seen as either part of the software engineering or systems disciplines rather than PLT.