When it comes to the cables, the only important factor is how much of the signalling light is lost through the cable, and how much external light can creep in through the connectors etc.
Frequency is absolutely NOT a limitation for the cables. Toslink works with red light at a wavelength of 650nm, which is another way of saying that the light’s frequency is 461.2THz. That’s 461,200,000,000 kHz.
The component limitation I talked about in the previous post is to do with how quickly the semiconductor pieces connected to each end of the cable can switch between on/off states when converting electricity to light (transmitting) or light to electrical conductivity (receiving). The cable needs to pass as much of the transmitted light through to the receiver as possible, while avoiding allowing extraneous light to enter through the connectors, so that the receiving semiconductor can accurately track the on/off signalling of the transmitter.
Plastic cables lose light strength at a rate of approximately 1dB per meter. Glass cables lose light at a rate of a fraction of a dB per KILOMETER. By losing less light in transmission, glass cables can provide the receiving semiconductor with more light to help it more accurately track faster switching between on/off states. That’s all.
With the right semiconductors for transmission and reception at each end, our common plastic TOSLINK cables would support any conceivable audio data transfer rate, no problem.