Hm, there may have been a miscommunication/mistranslation between the other mod author and my mod, but the reason that this recipe causes the error, is because the original "swords-armor-heart" uses 5 "starry-armor" ingredients in its recipe, which all have equipment grids, which means that they can't be stacked in a single item slot. Reversing this recipe causes the 5 armors to become the results (the output), which is not allowed by Factorio.
However, I should have already had a method of detecting these recipes, and disabling the reverse-recipe, so that this error shouldn't happen. So in the end, it does appear to be something that I need to fix on my end, but not for the reasons stated by the other mod author. Will be back after some testing