Actually, that's not what it's saying.
The Claws power gives Weapon: 1 and Inhuman Strength gives a +2 to weapon damage.
"Use the stress bonus of the claws and strength as a layer on top of the sickle weapon it carries (Weapon:2 itself, upgrading to Weapon:5)," is pretty clear: it's starting with a Weapon:2 Sickle, adding Claws power Weapon:1 on top of that, and then Inhuman Strength to make Weapon:5. Yes it's stating that its "Claws" power is actually a Sickle, but it's stacking that with an Equipment Sickle as well.
If it was just the Weapon:2 claws and the Inhuman Strength that'd be Weapon:4