Flawed.
Why is there only 2 different timeline?
Who ever said there was just 2 timeline?
When they entered quantum realm, there was 3 different destination at 3 different time. Surely they all went to 3 separate timeline. Then Cap and Ironman went back to 1970. Thats a different timeline altogether.

In total, there was 6 different timelines involved. 7 if you include 1945.

Most of people theorize that we have multiple timeline like this one. The thing is this time heist base on Quantum Physics that it is possible to be in two locations at the same time—at least for an atom or a subatomic particle, such as an electron. For such tiny objects, the world is governed by a madhouse set of physical laws known as quantum mechanics

so i disagree with any multiple timeline theory. we can only have 2 timeline exist at the same time as per quantum physics. cannot have 3 4 5 timeline