"Digital RCA" cables are not at all the same as standard RCA interconnects. Theses are digital coaxial cables, which happen to unfortunately share the same plug. They are NOT designed to carry analog RCA signals! They are designed to have a 75 Ohm impedance, but should only be used for digital signals. Your vintage system is entirely analog. Do not use these.
The basic quality of the cables aside, simple transmission line theory which is well know and understood tells us that this is not necessarily true.
All coaxial cables, "digital" or not will have a characteristic impedance determined by their physical construction. See the picture below.
The characteristic impedance of a coaxial cable only comes into play when the length of the coaxial cable approaches or exceeds about 1/10 of a wavelength at the frequency of operation. For an audio signal of 20 KHz this would be about 3800 feet. And when the source impedance and the load impedance are the same. This is not usually the case with analog audio signals.
Even basic "analog" coaxial cables will have a characteristic impedance of from around 50 Ohms to around 100 Ohms. But because the frequency of the analog signal, the coaxial cable does not function as a transmission line and the characteristic impedance does not come into play.
Coaxial cables designed for digital audio signals will be designed to function as transmission lines, hence the specified impedance, but this does not rule them out for use with analog signals. In fact the opposite may be true.
The debate about the best "sounding" cables aside, analog signals are perfectly happy traveling along "digital" coaxial cables.
Again this is basic transmission line theory 101 which is well known and understood.
There is no smoke, magic, or mirrors involved.
BTW, you mentioned BJC products. Note that they recommend the same 75 Ohm coaxial cable for both analog and digital audio.
