This is indeed very outdated since solr.remote.url is the old way to configure remote solr. If you look at the linked reference documentation on https://extensions.xwiki.org/xwiki/bin/view/Extension/Solr%20Search%20API#HSetuparemoteSolrserver it should look more like:
solr.type=remote
solr.remote.baseURL=http://$INDEX_HOST:$INDEX_PORT/solr
You also need to make sure the expected cores are created (it’s much simpler on Debian-like distributions since there is packages for it).