Earlier quoted context omitted.
The thing is, much like "perfectly white lights" you can mimic the non-linear behavior of tubes with transistor circuitry. On the extreme end you can integrate a DSP into the line to add a "vacuum tube" filter onto the sound in pre-amp. Thinking you can't do that is like thinking all LED bulbs must be 5000k and only incandescent can give that warm glow (Which, funnily that color was chosen to mimic gas lights before…
But when you do that, you are still holding the tube circuit as the reference model. And what you built is more complicated; it has components that have no counterpart in the tube circuit. (In the digital case, you have hundreds of millions of tiny transistors, which are switching full on or off.)
Yea, but so what? It's cheaper and easier and you get the same effect, without all the downsides of tubes, like heat, warmup time and wasted energy.