I’ve read extensively on TOSLINK and I think the “15 metres” length is actually more typically “15 feet” for plastic TOSLINK cables. TOSLINK is NOT fiber optic cable specs and construction. It was invented by Toshiba (hence the name) as a cheaper, sturdier (but not too sturdy) alternative to expensive, fragile fiber optic GLASS cable (although there are glass-based TOSLINK cables).
The Wikipedia article on TOSLINK nicely summarizes the properties, including transmission distance without reinforcement:
There is a thread with my usual long-winded discourse on the subject: Does It Make Much Difference What Type of Audio Output Is Used for TV Streaming? - Analog vs. Coaxial vs. TOSLINK S/PDIF? - #17 by jim_lewis
But in the post that I cite (after the Edit_Update), I mention that I had an unnecessarily long cable (at least 10 or 12 feet) that I had put some tight bends into). After I “straightened it out” by putting the excess cable length into some big circular loops, I got much better audio output on my amplifier system. But I also inserted a powered TOSLINK splitter into the circuit, too. Powered splitters serve as repeaters, boosting signal strength (although Haggis in the thread points out digital transmission is often all or none). There’s a link in the post I reference to a very expansive website on digital audio: PCM Audio in Stereo and Home Theater