Bitcoin is very suited for this model because it's supply is predictable and very transparent.. Another contribution to this is the Halving where the number of new Bitcoins created in each block is cut in half every four years. This Halving process helps to give the stock to flow a higher ratio which means that Bitcoin is becoming scarce and so its value is increasing...
It's true that bitcoin has a total of 21 million supplies but how about the speculations that the total supplies won't be mined as a cause of users loosing their bitcoin? So as a case study to think this model could determine the future price of bitcoin just because it total supplies is notable, so in that case of the valid supplied figures not going to be an exact to the total supplies, how's the model going to determine the ratio?
Maybe that's where the model went contrary after it could give adequate results in the past years and currently misbehaving according to how the developer tend to project it to flbe able to give adequate results.
See, left with me, I won't fall a trap to be convinced by any technology that claims to predict the future value of bitcoin. Perhaps if there is even a tool to paraventurely determine the future values of bitcoin, that won't be different from every other strategies or technology that stresses on predicting the future price of bitcoin because non of the techniques or analysts could be guaranteed or regularly make it successful to determine the future outcome for a long term.
So, take that stock to flow model as a failed mechanism. If it works for you too then I expect you should be a mutil millionaire in bitcoin trading by now.