Yes, it is really necessary. CBP told them that and put a note in the system, likely. Next time, without I-131, they may be pressured to give up their green cards and rightfully so.
They're likely going to be forced to give up their status by 2028 if they don't establish their lives in the US soon and don't have I-131. They may be placed in removal proceedings if they refuse to sign I-407.