Enchant restrictions are wrong.
Seems to be the case! When initially released, the guitar could only be enchanted with Exciting and Garden. Question now is whether there are two versions of the guitar (restricted, and non-restricted) or if Nexon just updated all the guitars to be non-restricted. ...think I may have an old version that I remember being restricted - I'll have to find it and see if that's still the case (for "historical" accuracy if nothing else :D).
I think that is a reasonable assessment, I know my Orange/Gold Xbow from gacha doesn't have piercing, so I would think maybe there might exist 2 Electric Guitars as well.
Stats on an existing instance of a weapon are fixed and don't change with updates (like older ranged weapons missing piercing), but enchantability is checked through tags at the time it's attempted, and tags are based on the item class (so they do update themselves now and then). I checked to be sure, and it looks like they removed/lifted/changed the restrictions on all the guitar models.