Stupid question: why are they looking for a discord alternative, instead of a group chat/Slack alternative? What am I missing?

Because slack alternatives also have video and audio call integration (either with jitsi or something else) and are battle tested with companies.

Because Discord isn’t just for friend groups, it’s also for large open-entry servers open to the wider userbase.

Not sure about slack alternatives, but Slack does have topic/general servers that people can join. SInce its mostly business/work targeted, a lot of these are tech focused (like for k8s, or pytorch)

Slack is also paywalled to a laughable level.