It's not possible to get aliasing without a reference signal or reference frequency. A single purely digital signal, perhaps clocked or perhaps not, has nothing to alias against. So the claim has something to it!
Once you introduce a master clock, especially one clocking an ADC, then aliasing can and does indeed show up.
In the digital domain Nyquist is a normalised ratio. It is not tied to a specific hardware clock rate.
So it's perfectly possible - in fact dangerously easy - to generate waveforms with components that are >Nyquist.
It doesn't matter if the hardware runs at gigahertz frequencies or subsonic frequencies. In fact it doesn't matter if there's never any hardware at all.
Because there is always an implied sample rate of 1 x fs, and any signal which generates components of more than 0.5 x fs will alias.
I don't consider a "digital signal" to have any sort of implied clock or sample rate whatsoever. This appears to be a terminology mismatch between audio people and electrical engineers, if the rest of this thread is any guide.
Here is my view of things: along the time axis, signals can be either continuous-time or discretized/discrete-time. Along the intensity axis, for example voltage, signals can be either continuously-variable or discretized/discrete-valued.
I consider an "analog signal" to be one that's continuous in time and in value. I consider a "digital signal" to be one that's continuous in time but discrete in value; for instance, the output of a (asynchronous) logic gate.
Your definition of "digital domain" seems to be discrete-time, discrete-valued signals; for example, the readings out of an ADC or commands into a DAC.
The difference between the two would nicely explain a lot of the confusion here.
Once you introduce a master clock, especially one clocking an ADC, then aliasing can and does indeed show up.