There is a great deal of truth to this. One major retailer in LA on the West Side is telling new customers not to buy Giga, but instead to buy Kontakt since everything imports into it.
This is not completely true.
Right now, only VSL largely imports into Kontakt because of the simplicity of its programming. How well other programs import into Kontakt depend on how indepth the programmer took advantage of Giga features. The more indepth the programming of any native programmed library, the more difficulty involved with importing it into Kontakt or any other program (like Mach 5, HALion, etc.).