Are you sure that the .jar file you tried in the command is the Jar executable file available at the end of Download page?

In Arch Linux, the script used to build the installable package, downloads the "Sweet Home 3D executable jar (25.7 MB, same size)" and renames it from SweetHome3D-5.1.jar to sweethome3d.jar in the /usr/share/java/ path. So it should be the same file.