So I've read the short story it's based on several times (The Story of your Life by Ted Chiang) and it's more explicit about how it works. It's only possible to comprehend Heptapod writing if you can think like they can. Likewise learning their language also helps you think like them, which is also pretty true about real languages

Heptapods do not have a linear observation of time. They observe their entire life simultaneously. However that doesn't mean that they don't believe in free will or choice. They come to Earth because they know at some point in the future the humans will be important. Amy Adams' character in learning Heptapod also learns to view time non-linearly. It's not permanent and tends to come and go the more immersed she is in the language. That's how she knows the secret phone number to call the Chinese general, that's how she knows what will happen to her daughter, and her marriage. However since she knows it like a Heptapod does, she has no desire to change it. Like how Abbott doesn't try to stop his own death