Is sum of currents equal to capacitor current for NMOS Inverter with Capacitor as Load?
Use mesh current at the node between the load, the source of the fet and the terminal of C_L
\$I_L = I_D + I_{CL} \$
rearranging gets this equation:
\$I_{CL} = I_L - I_D   \$
So it doesn't make sense that they didn't follow their own nomenclature with the arrows.