It's not an exploit in that context, the mod that allows you to do it is doing exactly what it is designed to do, and it certainly doesn't involve fake players or any form of behaviour not designed to be part of the mod.Crafting the reinforced watering can without losing a heart is only possible with cross-mod exploits. I don't know what exploit allows it but it is either something using fake players (ought to be fixed) or item duplication which is something that should be fixed regardless.
It's far more of an oversight of the mod pack authors rather than the mod authors.