User blog comment:Jho5689/Fairy Tail 337 and onwards/@comment-182.18.208.5-20130622125947/@comment-7418318-20130623000149

Acnologia appeared at a not specified point in that timeline.

It might have been right before F-Rogue went back like you think, or it might have happened a few days after the GMG and F-Rogue simply needed those seven years to learn Dragon Manipulation Magic (without that magic any trip into the past would have been futile) and/or figure out how to operate the gate.

Either way, when Acnologia appeared in that other timeline has little bearing on the current one. Since in F-Rogue's timeline not a single Dragon made it through the gate, we have no idea how Acnologia might have reacted to other Dragon's presence.