"stealthy" as in "unlikely to get detected *by the victim*". Someone would have to notice and track down the connection to that channel in the first place, unless discord is actively looking for "anomalous" activity.
Stealth in malware is not absolute, it's merely a delay, and this is a rather effective way to obscure the communication channel, even if it does end up biting the creator in the ass when its discovered.
Certainly less obvious than the old IRC C2 setups...