jounniy

They are using an official statblock which lacks this ability. The only way the shaman can have it is if the DM added it themself. And since this is a prewrittem module, it really shouldn’t have such oversights, especially since newer, inexperienced DM‘s likely aren’t familiar enough with the rules to know this, which makes it at least decently likely that they’ll just assume everyone can cast stealthily.

For reference: There is a duell in the module where a shaman tries to secretly help one NPC by continously casting Cure Wounds on him while supposedly only helping him “stand up” after he intentionally let‘s himself fall prone. To spot this, a PC has to specifically declare checking for interventions a then suceed a DC 15 perception-check despite being within earshot…