My personal theory is that the Galactus Engine was impossible to defeat because it's native universe didn't have death, which would explain why when the universe died, so did the GE, however it's just a theory
And until we see more of Flash Racer he should win, but it'll be quite interesting.
The scene with, Galactus along with the Celestials and Proemial Gods, holding off the Galactus engine is one of the best depictions of what a cosmic fight would be like in deep space.
Yet despite what is stated on panel, it has been shown on panel that the true darkseid is multiversal. So regardless of if the black racer is a universal manifestation it has the power to kill multiversal beings.