That is correct. The only way to use the ICBM to control the center's crossover point is to add an amp to drive the center channel. You can buy one additional amp (for a total of two) and have two monoblocks driving the mains (with the center and surrounds still on the receiver's power and the ICBM only providing crossover control for the mains), or have one amp driving both mains and another in mono mode driving the center (with the ICBM providing crossover control for the mains and center).

You could get two additional amps (for a total of three) and do a couple of things: three mono amps driving the mains and center (with the ICBM in line with all of them) while the receiver drives the surrounds, or one mono amp on the center channel and two stereo amps each driving a main and a surround (with the ICBM handling all bass management).