Remember that the -18 target is rms.
No one is suggesting that the loudest peak should be at -18db.
If the average level is -18db, any loud peak that comes along can go up to 0db but will usually not clip. It's still a guess! You still might clip with a hot surprise! It's simply a good starting point if you don't know how hot the peaks might get.
It's not really fair to even think about it like a compromise either. Like "sacrificing 3 bits of resolution". It's literally using the dynamic range of the recording device properly.
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