Because Oda didn't suddenly pull a Tolkien and decide that he wanted to go and explore three different eras, the mural is pretty obviously a foreshadowing what will happen in the future, it's not a coincidence that it was shown to us in the final saga, before we would see a similar situation play out.
This is where the flashback comes into question, Oda specifically ties it to the present. The flashback starts with Rocks D Xebec meeting Imu, the man he dreams of taking down(well actually, it starts out with Harald meeting Ida, but you understand what I mean lol, the extremely plot relevant part), then we see him through the years trying to get Loki's father to eat that Dragon fruit, because it's one of the 2 fruits crucial to his plan to take down Imu. You literally need Loki's power, and we later see that at the end of Rocks's life, he, his crew, Garp, Roger, all tried and failed to do anything to Imu. Because that "overgrown Lizard" fruit is obviously essential to taking down Imu. It's not really an exaggeration to say that almost everything that's happened on Elbaf is about building up Loki, the only character on the alliance side who can reasonably do more then him is ofc Luffy, and in the overall plot, Blackbeard and Imu.
That's why I think it's borderline dishonesty, the mural is pretty obviously Oda's way of telling us that Loki is one of the most important members of the alliance, and the rest of the arc has just been explaining how and why.