I don't think Discord is a substitute for HN. If I know an exact community with a shared interest I want to join then yes, I can find their Discord an join it. But how am I supposed to come across that topic and become interested in the first place? HN (in its ideal form) is a place where I am introduced to a variety of random but potentially interesting technology-related topics, from programming languages to passion projects to lessons in business etc, some of which I will pursue and seek out a community for, and others which I only care about minimally, not enough to want to join a dedicated community for.

If you want to compare HN to Discord, I would say that HN is like joining 500 Discord servers that together comprise the type of content discussed and posted about on HN, and every day checking each one's announcements channel, and skimming through general chats. But you still have the issue of being introduced to new topics, for example if you're interested in the AI trends then your selection of Discord servers that comprise what's posted on HN would be dramatically changing in the past few months.

I've found Discord worse in every way. Almost every server I've been in is highly reactionary to any kind of accidental phrasing. Walking on eggshells is a constant process in any Discord server. It also descends into the same type of problems as any other chat services as it encourages short messages rather than long well-thought-out posting.