Do you have any link to some official msys2 documentation stating that the "posix" variant is default?
It really seems to be the case:
https://github.com/msys2/MINGW-packages/blob/9f766f74cef87296b08c4508e12f600...
But the libstdc++ coming from the debian cross compiler differs from the libstdc++ coming from mingw. Not really sure what's going on. In any case we will probably have to compile libLerc by ourselves unless someone has a better solution.
So Github user @chadchoi was right. They said it's not thread models mismatch but libstdc++ versions mismatch. As I know, you can't upgrade your cross compiler because Debian is Debian, they never provide up-to-date packages. Building your own libLerc is the only solution.