The sc3-plugins-3.10.0-Source.tar.bz2 file that's part of the https://github.com/supercollider/sc3-plugins/releases/tag/Version-3.10.0 release is a gzipped tar, not a bzip2ed tar.
The commands in the build script are incorrect in that they use -z which creates a gzipped tar but name the file .tar.bz2 https://github.com/supercollider/sc3-plugins/blob/master/package/create_package.sh#L43
The initial script that was added in #186 was correct but #208 changed it to to this incorrect combination.