It seems that death eaters have "anti-time-looping wards" (chap 94), we don't know exactly how they work, nor if they were used in the graveyard, but it could very well be that no time-traveler can check the events that happened on the graveyard.
They were used. Voldemort mentions not completely trusting them.
Edit:
Quote from chapter 113:
I myself will guard this place until six hours have passed, for I do not fully trust the wards I have set; and four of you shall search the surroundings for signs of anything noteworthy.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 116.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)