Pretty old tutorial that mentions Gimp 2.4. I don't see anything wrong with your code (but Scheme isn't my cup of tea). However it seems that the command line in the tutorial is Linux-ish and you are using Windows which has pretty much the opposite conventions when it comes to quoting, so try to use outer double quotes and escape the quotes around the pattern:
Code:
gimp -b "(batch-unsharp-mask \"*.tif\" 5.0 0.5 0)" -b "(gimp-quit 0)"