I believe Dlanor's specific objection is things which are "unknown" (not invented yet/in 1986) or "hard to understand" (something reliant on an extensive amount of technical knowledge or extreme preparedness that don't make sense in the setting, or which would be incomprehensible to a reader).
A device to knock on a door is just a simple fulcrum, a recording is basically a tape. Things like Battler's suggestion of a wire or paper or something to pull a key under a door or a stake/bullet shooting trap in earlier episodes would also probably be a simple device, Beatrice just denies their use in those puzzles/deaths.
But yeah, Dlanor can just ignore any rule not convenient to her own side. She's not required to play fair if she's not on an impartial side. She doesn't seem to like playing dirty, but she'll do it.
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