It's not just about Big Mom here. He also talks about his crew. The two strongest characters of the crew after Luffy are Zoro and Sanji. Sanji is already there, but Zoro, the other power of the crew, is not. That's why he talks about not being at full strength. If you pay attention, it says Zoro and others. Luffy, Zoro and Sanji always fight the strongest. If you infer Zoro>BM from here, it means you don't understand what you read.
Although you said Zoro was stronger than Luffy.