It's not just a reference. The Forge on Ginger Island, forging your weapon has greater effects for higher level weapons.
Well technically.
Weapon level is derivative of the weapons damage primarily but also other modifiers like attack speed, etc.
The forge will sometimes have a greater effect on higher level weapons because some of the effects are percentage based.
Ruby for example adds 10% higher minimum and maximum damage. So on a weapon with 44-60 damage it would only chance by about an average of 5 damage where on a weapon with 80-100 the increase is about 9 damage on average.
Other forges like defence are point based and the weapon doesn’t matter, they will give a set amount of a value, usually more on the second forge of that same value, and that’s it. One topaz gives one defence. One emerald gives like 2 speed and the second emerald gives 3, the final gives two again.