Conjoiner fabrials and those variants would not require pewter. On the other hand, zinc and brass are necessary to control the strength of a fabrial. Augmenters are only a class of fabrials, which consists of fabrials having an area effect.
Does she say that Pewter is not a basic component? It's probably because you don't always require the attribute to be projected(Eg. spanreeds)
This clearly tells us that the pewter is what causes a heating fabrial to heat up. Zinc in this case would change the amount of heat/the temperature of the fabrial.
They do different things. Zinc makes the spren stronger, but it would not have any effect on it's surroundings. Pewter makes the spren project its attribute.
Chasmfiends jump?!
But we see skyeels hunting by crushing down on small animals. This would be much more difficult if they had very low mass.(Which they would need to fly)