Could be like the improvements seen when running applications using DXVK. My understanding is that sometimes these translation layers can use newer and more efficient methods than the path that a native implement for the time would use. I'm not a subject matter expert though, and could be completely off base.

IIUC there is no syscall translation, it's more like there are separate libc implementations and the correct one gets selected at program start based on the OS.

So like in-process WINE?

Yes, this seems fairly accurate. In fact I think Wine supports this mode where you link Wine into a "windows program" at build time to produce a "native" Linux executable. So basically the difference is that you target a POSIX API rather than Win32 and the backend implementation is selected at runtime rather than build time. But both projects have the same idea that they will implement a particular API on multiple platforms.