I will try to answer all your question, limited to my understanding of the model.
IMO articles like OP are irrelevant to the Bitcoin and other altcoins where prices are driven by specula and specula only.
1. The Assumption
Let's see about the equation
Price = 0.4*SF^3 translate: constant*SF^constant, the author is saying that the only variable matters in price/value discovery is Stock-to-flow. This is conceptually wrong since many things affect the price/value discovery. What if the world economy is in a recession? Is it a coincidence that there is no global recession in the past ten years.
Is it make sense to include: economic growth, inflation rate, risk-free rate?
Model R^2 is 95%: the model has a correlation of 95% with actual data. All other factors than SF, speculation included have an impact on the BTC valuation, but they account only for the residual 5% of the value. So, including them in this model (how?) only increases the forecasting capabilities of 5%. I think we can agree it's not worth the effort.
Regarding BigBoy89 observation I would add that the above consideration is for Bitcoin only. For altcoins I don't knwo what is driving the price, not SF for sure.
2. Model fit data or Data fit model?
Ideally, the researcher changes the models to fit the data. However, it's often difficult without "data treatment." Thus, in my limited observation, researchers tend to do the opposite, transforming the data to fit the model.
I'm one of the "don't transform your data" kind of guy.
All the data are publicly available and open to scrutiny on
PlanB github. material errors have been found in the past, and author has been ready to adjust his model accordingly 8e.g. Silver stock to flow).
3. Real-world problem
The data capture real-world dynamics up to 2019 (assumed that the market is efficient). Therefore, if the researcher creates a model about it, he will find a model that captures the dynamics, well... up to 2019. The dynamics will certainly change in the future, thus making the model invalid for forecasting.
As I showed above, analysing data before the first halving , data collected in 2009-2012, correctly predicts price in 2019. Of course dynamics can change after 2019, but I guess that if it held during the wild ride from zero to 10K, it is probably going to stay strong in the future.