Now why would you need 32KB of flash to control a soldering iron...
Probably some little thing like more space for a better control of the temp.
They might be doing something simple like watching how fast the temp is changing.
On heat up this could give the control program an idea of how much mass the current tip has and what adjustments to make to make control hold a temp better.
While soldering this could give a hint of what is being soldered like to 30 gage wires or something to a huge ground plane.
Or something simple as updating the control program while keeping the current control program functional in case something goes wrong during the update.
According to Renesas's site it's only a $0.10 to $0.40 cost difference.
C