BlivetWidget
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As for designers using it for "detect magic," that's just sloppy design and not worth emulating in my view as it is not in line with the rules. It's certainly in line with the rules of D&D 4e though (see D&D 4e Rules Compendium, page 135), but one wonders why it's showing up in a D&D 5e module. Sounds like whoever wrote that was conflating the rules from different games.
It's fine if you don't want to run the game that way at your table, but "whoever wrote that" was Wizards of the Coast. Specifically, the lead 5e designer, lead rules designer, and lead story designer all show up on the author lists of these adventures. They clearly intend Arcana to be used that way.