Difference between input current and supply current?
\$I_I\$ is the input current into an input.

Even though it isn't mentioned in the datasheet, I derive this from the fact that \$V_I\$ in table 6.3 is the input voltage, then it is "logical" that \$I_I\$ is the input current. This current is supposed to have a very small value which makes sense as this is a CMOS inverter and CMOS logic is supposed to have extremely small input currents.

\$I_{cc}\$ is the current flowing into the supply pin \$V_{cc}\$

Note that any current that is delivered (to a load) by the inverters also flows into the IC via the \$V_{cc}\$ pin. We want to exclude these currents so that's why it says: \$I_o = 0\$

The only current that flows in this test are leakage currents which are supposed to be small that's why there is only a maximum value listed.