As someone who participates in relatively high information density Discord servers (mostly ML related), search is rarely a problem for me. Yes, you need to do keyword search, but it's not the end of the world-- whenever I've needed to find a message 1-3 years back, I've found it successfully. Even in servers that I'm relatively new to.
I believe they meant "not searchable from an external search engine", which is a serious issue in many people's view (including mine). I would also suggest that it's a problem that chats are not browsable, because if you're new to a particular problem, you may not know what keywords you need to search for. With a forum, there are categories you can browse and usually discover the keywords you need much easier.
