for me the key is the black-hole, others may disagree but no one has ever used a black-hole in star trek to go back in time so this explains the alternate universe

any time that someone has gone back in time in any star trek tv show or film they have gone back into their own timeline that they can alter and mess with

the nerada and prime spock went through a black hole and went into a quantum universe that was identical to the one they where living in but everything they did from there on created an alternate universe, there are meant to be infinite numbers of these universe and are created when ever someone makes a choice as seen in the tng episode parallels

so for example if picard now jumped back in time to 2230 or even 2360 he would experience the timeline that he has read about. ie no nerada. but anything he did their would affect his timeline
even if he used a black-hole he would still go back into his own time but would now split into a brand new timeline leaving the original intact

in reality it does not really add up but as its a tv show you have to give them some leeway