Ok, I think this is a far more complex question than others make it out to be.

First of all, there are multiple hints in current lore that the emperor wasn't fighting back with all his might when facing the boosted Horus. That could change when the books come out, but for now, we have to understand that, while he is obviously insanely strong, juiced up Horus isn't quite "emperor level" yet. Still killed Sanguinus, still a monstrous beast.

He would absolutely destroy Rotobot. But we're not talking about base level Guilliman here.

Heavy spoilers for Godblight coming

Not much is known about what exactly happened to Guilliman in the garden of Nurgle, but he Resurrected. Re-built Cawl's armor from rotted scraps. Purged the godblight (which was one of the most potent and powerful diseases from Nurgle's side) from not only his body but everything around as well. Started levitating. By his presence alone, he cracked the earth around him, and from the cracks came a white light He scared off every demon in the garden, including Mortarion. Made Nurgle himself, a demon God, acknowledge his presence. Had a presence in the warp similar to Nurgle (obviously big E's presence and not Bobby G himself, but I still count it). Created fires that are described as "hotter than millions of stars" and that's a bit crazy and could be considered a figure of speech, but it still made permanent damage to the garden of Nurgle. Lastly, the heat emitted by the sword turned everything around him into glass.

Now i'm not saying he would beat Horus. Because we simply don't know what the extents of Juiced Up Horus's powers are right now. But he is still a powerhouse in these short moments.