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.
I disagree that compiler analysis/optimizations would be considered under 'systems', I would expect that kind of work to be published in PLDI, which is definitely the PL community. Systems is more OSDI/NSDI/SOSP.
Metaprogramming also has a rich history in traditional PL venues like POPL and ICFP, such as the Scheme/Racket work on macros, Template Haskell and friends, staged computation work a-la MacroML.
I say that I'm a systems person to signify that I want to talk about ASTs, IRs and compiler output rather than core language models and formal semantics.
But you could argue about definitions all day.
The relevant observation is that the reason this list focuses on formalisms like type theory and models we can reason about formally like FP is that this is what we mean by PLT.
You mean, systems in the OSDI/SOSP sense? Systems and PL implementation (compilers) communities have always overlapped quite a bit, maybe more so a decade ago than today (e.g. see Sanjay Ghemawat and Jeff Dean as PL people who did systems who are now doing machine learning...).
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.