What did Emperor Qianlong mean when he said that the lamp holder was facing down?

This is a political satire. In the past, candles and oil lamps were used for lighting, and the lamp heads were all upward. In order to refuse the tortoise's request, Qianlong deliberately said that the lamp holder was downward (in his view, this is impossible), but the lamps used now are downward. Are you kidding? So now the tortoise is an official (an official is a tortoise).