The metric to calcul the accuracy is directly done in your browser. The javascript is not obfuscate. We are comparing the distance between forecast value and market value.(the market value are update every hour)
Now, the knowledge you are referring to, apply for classification. Not for forecast. Overfitting and underfitting are not a concept you can use to predict future value. I invite you to try to forecast future value to understand the phenomenon.
I invite you to try the product and not speculate on it.
Ps: the model can spike to 99% accuracy but some dip are not predictable
Thank you for your interest