This is happening because Slackware uses the original xdg-utils package, containing xdg-mime, where as other distributions (those based on Debian, at least) have modified the command.
The original xdg-mime, and the one in Slackware, uses the command 'file -i' (absent KDE or GNOME) to determine the filetype. This mostly looks at the contents of the file, not the filename. It does NOT use the shared MIME database at /usr/share/mime/. Therefore your compressed SVG file is recognized as a gzip compressed file (which it is, on the surface).
One of the changes Debian made to 'xdg-mime' is to have it use a command 'mimetype' (not present on Slackware) instead of 'file'. 'mimetype' works like 'file' but queries the shared MIME database, not the 'file magic' database. The shared MIME database bases its answer on the filename, not the contents. Therefore your *.svgz file is seen as compressed SVG.
In my opinion, Debian's fix just moves the errors somewhere else. For example, take a JPEG file and rename it to test.png. On Slackware, 'xdg-mime query filetype test.png' correctly identifies it as JPEG, where as the same command on Xubuntu believes it is now a PNG file.
Hope this helps explain it, if not fix it for you.