“Dispersal” is the back side of blue and black Spell split card "Discovery.
When opponent have full hand,
first part “Return the opposing creature with greatest cost to its owner’s hand.” does not work and second part “Then, your opponent discards a card” is working.
As there are no condition “If your opponent have 5 or less card, …” stated in the card text, I believe that “the opposing creature with greatest cost” should be destroyed, like other “return to hand” effect.