It's not open to interpretation. The people who made the film said its a branching timeline that co-exists with the original one. So that's what it is.
But, if I remember correctly, they only said that in interviews that were conducted a few days after the film was released. To me, the film, taken on its own, seems to indicate the historical reality, the timeline itself, has been altered by Nero's arrival, reshaping the original reality so that no players involved, including the original Spock, can correctly anticipate the outcomes. Nero tells the younger Spock that they don't know each other ... Not yet, indicating Quinto Spock is the younger version of original Spock.
I think the "altered reality" line was probably made to be vague in the film script, so that it could be interpreted multiple ways, depending on how audiences reacted to the story.