Synthrapol suspends unset dye particles in water so they can be washed away instead of settling into other fabric. Retayne sets dye. I think you were really lucky that your fabrics didn't bleed. If they had, the Retayne would have permanently set the bleeds.
I always do my first wash of a quilt in Synthrapol. Retayne is most suitable for individual fabrics with a tendency to bleed, as it will usually (not always) stop the fabric from bleeding more.