In 1st quarter of 2013, the price of bitcoin was $135 so if it was over 286BTC, the calculation would be 286BTC * $135 = $38,610 approx. But the ending price for that year increased to $700.
While for 2018, the bottom was $3,300 and the calculation is 8.4BTC * $3,300 = $27,720 approx. I don't understand the metrics and price range that the chart took if there's a specific price that he has used as basis. And as for 2023 predictions, due to that chart the price of bitcoin obviously increased so what if around 2-4BTC?
The price used was the average of every single day's price during the specific year (comes from CoinMetrics.io). So 2013's "bitcoin price" was the average price of Bitcoin across every day in 2013.