^What they did was start a new timeline... Although, I can't understand how effecting the new timeline made a difference in the one they had inhabited... Unless, of course, pass Thanos ended up in their timeline, but the Thanos of that timeline was deceased... So, it was an "alternate" Thanos who learned of the events, just as it was an "alternate" Gamora who gave zero damns about Quill or the GOTG.