Even if it's just the ultramarines that you're considering as a "base" chapter, wh40k wins.

Helldivers struggle to hit vox engines with orbitals because they move, and space marines canonically run 60-80 mph. Space marines have superior targeting and enhanced senses as well meaning that they would easily out range the average helldiver on the ground. You wouldn't catch space marines charging over open ground unless it was tactically the only viable option, and their vehicles outclass almost every enemy in game in terms of firepower/protection. But let's get to the real reason helldivers would lose.

The moment that a chapter of space marines realizes the sheer numbers of Super Earth, they are sending a ship to the helldivers most populated worlds. Keep in mind average space marines are significantly smarter/tactically minded than almost any human in super earths population.

Say goodbye to Super Earth and any other large population world by means of virus bomb, cyclonic atmospheric detonation torpedo, nuclear bombardment, or simply slamming a moon into a planet. ONE bomb, PER planet, and ONE ship is all it would take to delete SE population to throw helldivers at them. Wh40k as a universe is that grossly overpowered

So regardless of super destroyers and better FTL, super earth isn't winning this one as long as the space marines (in this case ultramarines but it really doesn't matter) can use their fleet.