It does, yes. Desperate Power doesn't change the core function of Ryze's spells, it just causes them to also deal spell type AOE to all units near the target in addition to the base effect.
On a similar note, do karthus q and morde q count as single target for the purposes of spell vamp, rylais, and now muramana? I ask because they behave differently based on whether they hit a single target or multiple targets.
Yes, they are single target spells when they strike one target.
Muramana Ryze
Yes, they are single target spells when they strike one target.