At this point, SHIELD has done more monkeying with time than Endgame did. And they didn't even need any stones.
I am still trying to figure out the timeloop - they were able to change the future and get back to 2018 because frozen Fitz helps them in that future to get back.
Now if they go and find him and he never gets to the point 75 years later to do what he did, then would they be able to change that future?
I guess its the same logic in EndGame - and probably wont even be addressed - much like the Flash TV series doesn't even bother explaining.