> What this all tells us is that we can watch the relationship between Bitcoin’s price and difficulty to see situations where Bitcoin is overvalued. These situations would be times where the difficulty rate stayed the same while the price continued to go up.

That seems backwards. Miners will adjust their costs until marginal cost equals marginal revenue. In other words, if the Bitcoin price goes up and the hash power does not follow suit, that doesn't mean Bitcoin is overvalued, it means that there's an opportunity to profit from running more miners.