Here are some things you could do:

1. Make sure the which.exe being used is the one linked in the article and not the one included in Git Bash, Cygwin, etc. Running which iproxy should show you something like D:\libimobiledevice\bin\iproxy.exe instead of /d/libimobiledevice/bin/iproxy.
2. If it still didn’t work, try moving the which and libimobiledevice executables to flutter\bin.