I did a little digging in the APK.
The Android Java code is packed using Qihoo "DexJiagu" into a shared library to prevent easy decompilation.
They are using the PCSX2 game database with its configs, plus a supplemental one with a handful of hacks to make games work.
Looking at one of the libs, it states that something currently crashes on "Linux 64-bit running in 32-bit mode", which may explain the lack of Shield TV compatibility.
Incidentally, sideloading it to a Shield TV from your phone should work assuming you've purchased it and are signed into the same account on both devices.
At this point, I can't tell if this is a fork of PCSX2 or Play!, or something written entirely from scratch.
The only thing tying it to Play! is the fact that Play! uses a file called "games.db". There's a file in DamonPS2 that lists that as an asset, but it doesn't seem to be in the app.
Also, the actual core of the emulator seems to be compiled with an older version of the Android NDK, one with GCC 4.8. Currently, Android uses some version of clang, which is found in one of the other shared libraries.
I don't have Ida and I don't really feel like doing any deeper digging so I'm not going to attempt to examine the shared libs further.