Mark Hamill says it’s not official.
Mark Hamill wouldn’t know.
he would know as much as any of you, which is practically next to nothing. so yes, i agree, he isn’t the final word, but i am very skeptical about that poster.
The difference is he probably just took a quick glance, thought “I haven’t seen this before,” and assumed it was fan made. If you look close, the poster includes images that are clearly from IX’s promotional photography and wouldn’t be available for fans to photoshop.