You are confused. xz does not provide the ability to find individual files inside of the format: it is not at all comparable to zip files, and like the other formats mentioned it compresses one file. The "seekable" property is that it lets you somewhat efficiently decompress arbitrary byte ranges inside of the file, which is why a compressed disk image (as used by your parent commenter) benefits from this property but the industry-standard usage of compressing a tar file (which is a file format which inherently makes it impossible to find individual files without reading the whole thing) totally throws away this benefit.
A tar file doesn't have a central directory like a zip, so you do have to search all the file headers. However, each file header contains the length of the file it describes, which lets you seek past the content of any files you don't care about if the tar isn't stored in a way that would prevent seeking.
Yeah: what they mean by "container format" here is similar to the usage in video compression file formats, where ".avi" doesn't imply any particular compression algorithms. An xz file is a container format which is designed for use to store another container format.
