You are mistaken. Filling an oversize hole with a dowel and then drilling it out for the tuner leaves a "collar" of dowel drilled through the end grain that the tuner sits in. The grain direction in this "collar" means that wood will be far more compressible than if the grain runs in the same direction as the neck/head. The thicker the dowel collar, the more compression is possible. I'm not saying it can't work at all with (maple) dowel, I'm just saying it's definitely better with the plug grain lining up with the head, especially with the tensions from bass strings. In addition, if you can see any of the repair, it looks 1000 times better with a grain-matched plug (that's an actually carefully measured and quantified result: 1000 times).