When you use it, you get a nice Korn shell and it is built on PE binaries linked against PSDLL.DLL. there's a functioning but very old version of GCC that ships with it.
The PE binaries mark up the desired subsystem to be invoked so you don't have to be in the environment to execute one - the kernel takes over.
PSDLL acts as a translation layer for NT much as kernel32 does for win32. You can't run unmodified Linux binaries like you can with wsl. On the other hand, WSL requires that you invoke lxss with some special com magic to get access to Linux first so you can't just exec an elf file directly. The Pico processes you mentioned - these allow the kernel to install specific handlers/translators of their syscall functionality into the windows kernel.
So yeah architecturally they're pretty different and WSL isn't really the same subsystem concept they started with. On the other hand it that's probably a good thing because everything needed a rebuild for SUA.