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You can really tell that they wrote parts of the module before publishing the rules…
(ttrpg.network)
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…
I mean, I don't see an issue with hiding the casting of a spell from people. I do think it should probably be the other way around for players (do a slight of hand to hide the semantic/material components, and beat passive perception), but in principle I don't see why you shouldn't be able to hide those?
Because it would make subtle-spell kind of redundant and why even use components in the first place if they cannot be used to tell you are casting something?