Exactly.
Piccolo was evenly matched with 17 after fusing with Kame. After that fight he went into the HTC and trained for a year for the cell games, so doesn't it make sense that at that point he was stronger than 18? Vegeta went from getting smoked by 18 to being able to handle imperfect cell with no problems. Obviously Piccolo isn't a saiyan so he didn't improve as much, but even improving a fraction should put him above 18. On top of all that, Piccolo has been keeping up his training. He says so himself when he chokes out SSJ2 Gohan in super. The show doesn't explicitly state it, but I think there's enough evidence to suggest he is stronger.
Not sure why I even care. I don't like Piccolo like that