Oh, okay: it does now, after I cleaned up after myself by adding
mod_gzip_item_include mime ^application/xml$ mod_gzip_item_include mime ^application/atom\+xml$ ExpiresActive On ExpiresByType application/xml "access plus 30 minutes" ExpiresByType application/atom+xml "access plus 30 minutes"
.htaccess file. Oddly enough, when I tried the other Google-recommended way of telling mod_gzip to add things,
file \.xml$, it ignored me. Dunno why.