I would be careful about saying that a steel sled needs no additional weight. The design of the sled to allow this must be such that the bulk of the weight is below the router otherwise it will tend to spin and can cause issues. I would say that using a steel sled reduces the amount of extra weight needed, but unless you design your sled like this you are likely to need some weight to keep it vertical. Note that the steel used in that design is 10mm thick.
The metal maslow also uses a c-channel z-axis which does contribute weight to the bottom of the sled, but I found that this was not enough to keep the sled oriented properly and to counteract the force of the bit driving into the work piece.
Edit I did look back a the metalmaslow design and it is bottom weighted. I also saw this in their initial descriptions, though I don’t know if they’ve designed away from the need: