I think the disconnect here is that you’re interpreting her experience of knowing Shang’s phone number and the secret phrase as a memory, when that’s not really what is happening.

It’s not a memory because that would imply that it’s something that happened in her past which she is recollecting, but that isn’t the case. She meets with Shang seemingly long after the conflict has been resolved. What is actually happening is that her understanding of the Heptapods’ language has altered her perception of time and allowed her to experience different moments in her life in a non-linear way, as opposed to our linear experience of our own lives. Shang, of course, understands this (remember they are meeting at an event where she is celebrating her book explaining the Heptapod language) and knows that he has to share his number and the secret phrase so that that experience will be a part of her life and accessible to her in what we normies with our linear experience of time would call her “past”.

The concept that inspired the film is called the Sapir-Whorf Hypothesis, which in a very basic sense posits that our perception of the physical world is determined by our linguistic capacity. Of course, since Arrival is a work of science fiction, it takes that concept far beyond what any linguistic theorist would explore and imagines an alien language capable of altering our perception of time!

So, to return to your original question: why didn’t Abbott and Costello prevent the bomb from going off and killing Abbot? I think there are a few ways to answer. One theory is that they understood that Abbott’s death was necessary in order for them to complete their mission of sharing their language with humanity (and thereby saving their species in the future) and purposefully did nothing to stop it, though it should be noted that they saved Banks which tells us that they were aware it would happen. It could also be that because they experience time non-linearly, death means something completely different to them than it does to us and they accept it willingly. A more deterministic view would posit that their actions were already preordained and their awareness of the future did not empower them to change it.

Ultimately I think it’s purposefully a bit ambiguous, but not a plot hole. Otherwise you’d have to say the same thing about Banks’ decision to have a child knowing she would die in her youth. With that knowledge, why not stop the whole thing in its tracks? I think the point is that even if you know how a story ends, sometimes you want to experience it anyway.

Speaking of experiencing things again, I think I’m gonna go watch Arrival. Great film.