But let's not get stuck on this point. There's a dozen other reasons why Discord shouldn't be used for technical forums
But as for why Discord is a bad fit - it's completely opaque to search engines, it requires an invite, it has a fairly complex UI and it's basically "unstructured chat". Threads are an afterthought and don't come close to giving any proper structure.
I want chat for people that, you know, want to chat - but structured posts with topics and categories are needed for any sane, long term knowledge-base.
People on chat (this goes for IRC as well) will continually ask the same questions and you will continually have to answer them again because there's no structure.
Every external chat room logs service I've seen has also had bad SEO, buggy, bad UI/UX, etc..