The confusion comes from the difference between "rebels" and "Rebel Alliance". The first is people rebelling, the second is an alliance of groups of the first. The Rebel Alliance as instituted by Mon Mothma and Bail Organa was started roughly 2BBY. However, there were rebel cells all over the galaxy since the formation of the Empire (to be pedantic, they have been around a lot longer, but thats not applicable right now).
Pamlo: We must scatter the fleet. We have no recourse but to surrender.
Organa: Are we really talking about disbanding something that we’ve worked so hard to create?
Raddus: We can’t just give in.
Vaspar: We joined an Alliance, not a suicide pact!
Rogue One (2016)
The TV show Star Wars Rebels follows the actions of a single rebel cell on the planet of Lothal. For the vast majority of the show, this rebel cell is only concerned about Lothal, its not until the very end that they join up with the galactic wide Rebel Alliance.
Saw Gerrera is another example. As seen in Rogue One he ran his own rebel cell that was not affiliated with the Rebel Alliance.
Mon Mothma: Yes, but Saw Gerrera’s an extremist. He’s been fighting on his own since he broke with the Rebellion. His militancy has caused the Alliance a great many problems. We have no choice now but to try to mend that broken trust.
Rogue One (2016)
So to answer your question in the title, the Rebel Alliance was founded around 2BBY (we don't have an exact date yet in current canon). However, "the rebellion" you reference in the body of your question is almost certainly a preexisting rebel cell that, pending survival, would have likely eventually joined the Rebel Alliance 8 years later.