The nature of time and space you see, is like a block, or a cosmic film reel, all the frames in the film reel exist and are equally real/existent/important.

Joe, the film character, in one frame, his mug of coffee is on the table, he is looking at it, his thought bubble shows that he anticipates drinking it, a few frames later, Joe is holding the mug to his mouth, and the coffee is tilted, next couple of frames ahead, the coffee is in midair, touching neither the mug nor his mouth.

A few frames later, the mug is on the table, and Joe's thought bubble is of him reminiscing about his experience picking it up, drinking it, and putting it down.

However, did you notice something?, none of his memories are true, time never flowed, he is simultaneously drinking it and thinking about it before and after, but if someone in the film were to tell him, he would scoff, it makes no sense, he knows he drank it, and that time passes, right?.

We are in the same situation, all instants of time are just as real, but we're inside the film reel, and thus subject to these technically fake memories, the problem is, our language only has words based on past, present and future tense, when I say "all of time exists", the normal sense of the word doens't make sense, the word exist implies the frame we're in.

So, saying "does the colossus of rhodes exist?" is, by the misfortune of our language, like asking "is the wall in the same place as me?", however, saying "the colossus of rhodes doesn't exist now, it's behind us a couple millenia ago" is the same as saying "the wall is not here, it's over there, a couple of meters behind me".

Now that we've explained time in a very odd way by trying our best with the words, is this scenario possible?.

No, anything we do will be done, is beind done and already has been done, from a certain perspective, if the AI did anything big, we would notice it, if it didn't do anything big, this means it probably hasn't done anything that would help it's goal, not to mention that time travel would require warping spacetime in a certain way, and this requires negative matter/energy, there is no indication this actually exists or is possible to create.

Plus, it's a really dumb idea to begin with, why would you go back to ensure your own existence when you're already permanently fixed in the spacetime film reel?, you couldn't change anything about it, only fulfil things that already happened in the past.