1) That was a typo. I updated all data in the chart, except I forgot to update the formula parameters in the chart... Updating the chart was a hard manual process, and so subject to minor errors.
2) The chart should be updated regularly...
3) I made those charts to have an idea of whether the BTC price was overvalued or undervalued at the moment of every update. The chart could also be useful for estimating under-over valuation for one or two years in the future. And also gave and idea of the enormous revalorization of BTC that was left by them. But I never did that as a prediction, because no one can predict the market behavior. The price hitting 10k with just days of difference... is just luck. No cristal ball here... Seriously.
4) The wolframalpha gave an approximation of the value at a given date, given that updating the chart was a hard process.
Glad to see the legendary Trolololo!!
Have you ever added overbought and oversold bands to better visualize those levels? Is there a model that includes bands that you like more?
It would be interesting to see where the overbought band is right now or 6 months ahead.