But the events in 1977 were not happening at the same time as the events in the present day. Those events had already happened at the time of the pilot. The very fact that the hatch, the Swan station, existed is proof that those events had already happened. They even called it the incident before we saw the incident. The reason that the hatch existed and that they had to push the button was because of what the the time traveling losties did when they went back to 1977. They didn’t change anything, they just actualized what we already knew happened.
Things like the Hatch and the incident. Or Faraday telling his girl to leave the island. Or Faraday being shot by his own mother. Or Miles’ father losing his hand. Or the Volkswagen van with the dead guys. Those sorts of things had all already happened and we were just seeing the events that had already happened in the past. The losties did not change anything by going back there.
There is only one storyline in LOST, one history of events and just past, present, and future, no sideways into another word or dimension like in ONCE. I understand they called it a sideways, but they were still only flashbacks and flash forwards.