I think he had 3 pieces, went to check the wheelie bag, agent processed it, he realized what happened and siad he would take it on board. Agent said 'no, you can't have 3 pieces'. Then he said he could put the laptop bag inside the suitcase to make it 2 pieces. Agent said no for some reason and he had to check it. He then removed the laptop bag and travelled with 2 hand luggage pieces.

Assuming that once the laptop bag went into the suitcase, the suitcase still fit the sizer (not clear when he put it in the sizer), agent was wrong. If it was too big once the laptop was inside, then the OP did need to check it.