I think the goal here is to first launch a server using the --start-server command (successful) and then use the same executable to launch the client on the same machine (failed). Problem is they are using the same log file (or other shared file) which gives that error.

Solution is to use the -c ...

1) Updated factorio
2) Created a new map
3) Created .bat file like the wiki says

cmdwindow opened correctly!

4) Launch factorio in steam "Process cannot access the file because it is being used by another process"

Is this me doing it wrong? If so then the wiki needs updating.