Dear Agatha,
the sine wave provides the RoF frequency. However, the Quadrature modulator offer an intermediate frequency for the OFDM subcarriers. You may check the RF spectrum analyzer to view the spectrum of the OFDM signal. The designer chooses these frequencies. There was limitation on the available equipment to conduct the experimental realization of this design. That is why those frequencies were chosen based on the equipment.
Attached is a paper that summaries the work presented at CLEO2021.
Regards,
Ahmad