The article you link states 'Variety of placement and number [of coins], including but not limited to a single coin in the mouth, is characteristic of all periods and places.'.

So it sounds to me like a 'not always in the mouth, not predominantely on the eyes, but anything goes'

Edit: Not saying the title is wrong per se just a bit too decisive IMO.

Further edit: At least according some superficial online search it has been proposed that coins over the eyes could kind of combine the purpose of pay forcthe ferryman and just to keep the eyelids from opening but i didn't find many sources neither in support nor opposition of this theory.