I think it's a disservice to the Forge to downplay the overall benefits it has to Cultivation, even if it is 'only' the Farmer's Step while playing up the benefits of the Heart. Both are good, and the wording is both 1) Vague for both & 2) Not set in stone.
One (potential) inference could be that given the Heart has two distinct functions (Monsterform + Integration bonus), that the charm's overall power is split between those two.
Whereas the forge has (atleast by the admittedly vague wording) seems like it has one function, so theoretically the entirety of the charms power-budget is in that.
Look; I get it. The sarcophagus is actually pretty weird; I agree and if it had a different form I think it'd be a perfect charm.
However; one of the things that I like most about the Forge compared to the other charms, and I don't think is maybe getting enough acknowledgement (maybe it is, or maybe others don't care) is that the Forge is not KZ exclusive.
It lets KZ give back to the family that gave so much, literally their future ascendency in the form of Shuren's imminent Warlord's Step attempt which would've catapulted the Kongs into the upper echelons. Miss me with that copy+paste Iron Man arc reactor expy that is literally everywhere.
EMBRACE FORGE. REJECT ALL OTHERS