Entertainingly, the backronym for Slack is “searchable log of all communication and knowledge”—I kid you not: https://twitter.com/slackhq/status/875547942454689794?s=21

Agreed; Slack search is next to useless for me as well.

However, what is meant here is that content in Discord isn't reachable via search engines such as Google. Whole communities are invisible to traditional search indexing and only reachable through either a special client or an API, both of which require an account and empower gatekeepers.