as explained by @Jylpah above, the svgz sample used in the test is not a svgz, actually just plain svg with svgz extension.
I opened the sample file with Inkscape an re-exported as Plain Compressed SVG (.svgz), after re-export, the file info:
file ./sample-compressed.svgz
./sample-compressed.svgz: gzip compressed data, from FAT filesystem (MS-DOS, OS/2, NT), original size modulo 2^32 13794
CMIIW, compressed svg is part if SVG 2.0 draft specification, not even shipped in chrome?
https://chromestatus.com/feature/5760616295825408
https://svgwg.org/svg2-draft/single-page.html#conform-ConformingSVGServers