I mean, I get why Mike would be a bit put off by a ADTOE shirt, but I think this illustrates fully the differences between the viewpoint of MP versus the viewpoint of DT fans in general on the MP/DT split. In the mind of Mike Portnoy (I would guess, based on his comments on the matter) he views post-MP Dream Theater as a distinct entity from the previous incarnation of the band. But in the minds of most DT fans, it's just a new period in the band's history, not a different band entirely. While I wouldn't put it past someone, given Mike's acrimonious relationship with the DT fanbase post-2010, to wear the shirt as dick move, I think the much more likely explanation is that the fan just figured "it's a Dream Theater shirt" without consciously thinking that the album represented on the shirt did not involve Mike. He just wanted to wear a Dream Theater shirt to an ex-Dream Theater member's book-signing.
Again, I get why Mike would be a bit upset about it, but he should really give the guy (and fans like him) the benefit of the doubt in cases like this. After all, the guy was at a Mike Portnoy event; he is obviously a fan of Mike as an individual, as well as Dream Theater as a band. There is no reason the two need to be mutually exclusive.