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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…
But that's my point, 5e is already not a rigid system.
It is in this regard. It might be poorly balanced, but there are rules for things like the sound a spell makes or the specific spells available to NPCs. There are a lot more rigid systems, but DnD definetly isn’t on the casual side either.