The SDS lists an approximate breakdown(with CAS numbers):
We can deduce a few things:
The 57455-37-5 appears to be pigment.
This is not really a clay, it is just mica and cellulose with plasticity.
It needs not explode on heating.
Mica is permeable to water passage and obviously impervious to flame (but cellulose is not).
CAS# 9005-38-3 is a gelling agent, that must be used to impart plasticity and enable workable material with high water content.
It is not obvious why salt water is used, perhaps as a flame retardent (to hinder burning).
Hold-it clay usage information (from the manufacturer)
Video of Hold-It clay in action soldering jewelry components in place