Seventh Generation uses a plant-derived surfactant for its fabric softener whereas conventional fabric softeners use an animal-derived surfactant. Although the chemical structures are different, the two types of fabric softeners act in the same way. Thus, we would expect our fabric softener to reduce the capacity to absorb water the same as a conventional fabric softener.
In neither case would we expect the loss of capacity to be very large. However, we have not performed any experiments to verify this.