What mysterious property is a "true valuation" and how does one measure it?
The very definition of "valuation" points to the underlying "true" part -- valuation means an attempt to guess at the value of something.
It might not be possible to have a perfect guess (because nobody has all the data or knows all the future events that might affect it), but it's very possible to know a valuation if BS and critisize it.
Would you buy a Denny's dinner in rural Nebraska for 1 billion dollars?
So, yes, there actually are things that "no true scotchman" would do (else there's no reason to invoke a group named "scotchmen" at all).
In the case of OP, valuation is simply what the market will pay to own the stock of a company. Sometimes it coincides with what any individual will pay, sometimes not. There is no sort of magical intrinsic value to a company. All we are really doing is arguing over the definition of valuation.