I think Absolute Shadow is just what ultimately became of Darkness. It's not that people are mistaking the two, it's the Mashima replaced one with the other. Darkness hasn't been seen since he fought Mirajane, so he probably just decided on the Absolute Shadow form since it's less complex and easier to draw.
// I might have a slight idea of who this is because this was suggested to me before and even of it’s not this person I still have a slight idea lmAO but- anyway I’m actually certain it’s not that the demon form has ‘become’ Absolute Shadow. Mashima probably just needed a form he could make Freed use that’s less powerful than Dark Écriture: Darkness. Absolute Shadow is by no means weak but in direct comparison to Darkness it definitely and very obviously is. If it’s a change of design only then why change the spells name? Change the purpose of it? Change the kind of abilities of the form even? Nah son. On top of that the demon form is said to be forbidden AND in general Mashima hasn’t let Freed use ANY of the stuff he used during the Fantasia ARC because… hahaha, the Raijinshuu apparently aren’t allowed to get a legit fight :’) Babes are too badass. So yea, definitely no replacement but a ‘weaker addition’.