The backup mechanism allows an administrator to create a physically separate copy of index files and configuration metadata for a Solr collection. Any subsequent change to a Solr collection state (e.g. removing documents, deleting index files or changing collection configuration) has no impact on the state of this backup. As part of disaster recovery, the restore operation creates a new Solr collection and initializes it to the state represented by a Solr collection backup.
It’s probably safest to treat data in Solr as secondary data, in the sense that you should be able to rebuild the entire data set from scratch instead of Solr being a primary data store. I’m not a big fan of the author using the term “disaster recovery” instead of just “recovery” or “backup restoration” (as they’re different concepts), but it’s worth the read.