Optical brighteners aren’t the same thing as blue dye, even though they have the same role. Optical brighteners absorb high wavelength (UV) light and re-emit it as blue light. This helps prevent the new particles from overpowering or covering the existing dye. Blue dye would just paint the clothing more blue, which usually isn’t what you want.

So in this case it isn’t just marketing bla bla for some old thing, it’s actually a new(er) thing that replaced the old thing.