Discord search is actually great but you are right that the information is invisible to Google search and other external search engines. Likewise, at my company, a huge amount of knowhow is stored in internal chat logs. This sort of knowledge is the true "deep web" --- unindexable and unsearchable.
The information is also often locked behind login walls.
That accounts can be created for free (for most people) does not mean that the information is freely accessible.