He's referring to when Piccolo took their energy and attacked 4th Form Freeza when he was off guard.
Piccolo should have been close to full power before that seeing as he barely expended energy when briefly fighting 4th Form Freeza. Krillin should be in the same boat. Gohan I'm not sure about. He unleashed his full power against 3rd Form Freeza and never got healed by Dende, so I can see his power being much lower when he gave his ki away.