Channel Coding with Higher modulation
The first part of the statement says 2 information bits per modulation symbol. QPSK transmits two bits per symbol, so you don't have any opportunity to add extra error correction bits (channel coding) while keeping to the 2 bits per symbol limit. Using 16 QAM, you have 4 bits per symbol, so you have spare bits to add error correcting bits (2 bits per symbol for data, and an extra two bits for error correction coding). The gain you get by adding these error correcting bits can be more than the loss in sensitivity that you get by increasing the modulation order.