The blurb is:
Correct - it does suggest it will rebult the topic content.
Your reading of the second sentence, however...
It won't recreate missing topics - as far as it is concerned, as a result of a topic previously being created it now has a record of that topic's ID and the rebuild would rebuild the content IN THAT TOPIC.
Let's say you change the "topic display format" for some of the database fields...run the task, it rebuilds the content in the topic that exists already.
Let's say you don't have the setting enabled for a database...you have a load of records, you decided that you want to have topics too. You enable the setting, run the task, it builds the topics because none exist already.
But once the task is run....you have a topic ID. Delete that topic somehow...it won't be recreated.
The field that contains the topic id for a database is the "record_topicid"