Board Thread:General Discussion/@comment-26838855-20190803131136/@comment-1894786-20190807230416

Russos are sticking to the alternate timeline theory (thank god). Steve did not return to that moment on the bench, that's why he doesn't have the Quantum Suit on.

https://www.reddit.com/r/marvelstudios/comments/cn9lbx/were_joe_and_anthony_russo_directors_of_marvel/ew9238h/?context=3

A long standing theory is that Old Man Cap left his new timeline in 2016 when Peggy dies (assuming she still dies in 2016) and waited 7 years in the MCU. I don't buy that. I think, going by what the Russos said, Steve jumped to the main timeline sometime during the final battle or its aftermath, and waited until Young Steve left.

So that would make both official explanations right; Cap lived his life in an alternate timeline, but he returned to the main timeline when his younger self left, creating a loop for Cap's internal chronology.