You also can't change the timeline or you wouldn't get their to change the timeline.

They could change the future an alternate timeline, but not their own.

So if they went back in time to kill Hitler Thanos as a baby, then it would drastically change the way the future would unfold for that new alternate timeline. But when they went back through the quantum tunnel they would be back in their own unchanged timeline.

But then that doesn't compute with Steve going back in time and living his whole life with Peggy, which should cause an alternate timeline. But since he didn't come back through the tunnel it seems like he had lived in the main timeline and knew when to show up at that bench.