some kind of angel, maybe?

So what's going on here? Did Gerson exaggerate the effects of jealousy in chapter 5 for dramatic effect? Did we subvert the prophecy by somehow preventing the inferno of jealousy from happening?

No, the answer is far simpler: Lord of the Hammer isn't Deltarune.

Of course, I'm stating the obvious here, but the reason we were expecting an inferno in chapter 5 was because of Lord of the Hammer. But Lord of the Hammer is a retelling of the prophecy, not the prophecy itself. If we look at the things the prophecy told us about chapter 5 ("The Flower Man, trapped in asylum.") it's far more accurate.

So Lord of the Hammer just changed a few parts of the prophecy's story, and one of those changes was an exaggeration of the theme of jealousy in chapter 5.

And if we think about this a bit more, we really should have expected the inferno of jealousy to be a bit of a red herring: none of the synopses Gerson gives for the first four books of LotH are entirely accurate to Deltarune either:

Chapter 1 The March of the Dark King. The heroes defeat the king and stop the dragon.

Technically, all of this happened. But Gerson implies that the king was defeated before the dragon was stopped, and of course, in Deltarune the dragon is part of the heroes, which doesn't seem to be the case in LotH.

Chapter 2 The City of the Shining. The heroes do battle in chariots to save the Queen.

This just straight up never happens in Deltarune. I've seen arguments that this refers to the (optional!) Spamton NEO fight, as if Spamton would have won, this might have endangered the Cyber World residents, including Queen? But that explanation is a bit to convoluted for my tastes. I think the simpler explanation is that Gerson just changed Queen's role in the story from villain to damsel in distress, possibly merging her character with Noelle.

Chapter 3, The Isles of Northernlight. The heroes travel among the islands and catch a glimpse of a lost land.

This one is the most confusing one to me. We do travel among islands within Tenna's boards, but what would the lost land be? The original game? Mancountry? Whatever it is, Gerson definitly put a lot more focus on this lost land than Deltarune did during chapter 3.

Chapter 4, The Trials of the Holy Hammer. A great smith gives the heroes a terrible weapon.

This happens, but it's optional. Also, I wouldn't call the JusticeAxe terrible. It's not that good compared to say the BlackShard or even the PuppetScarf, but it is still the best axe for Susie in violent playthroughs for chapter 4 and 5.