Since there's no way to know how much hashrate is coming from which device categories, these are estimates.

Based on a slightly educated guess, it would appear they show too many S9s as decommissioned / offline in 2020, and only fraction of those resumed service in 2021. These devices were valued at $500 in the market (and for S9s that had failed, repairs were possible and usually economical). So that chart showing less than ~25% of S9s had been returned to service seems to indicate a poorly constructed estimate on their part.

Perhaps their estimate is based on self-reported numbers from the ASIC manufacturers. And even if those numbers provided for shipments was accurate, that doesn't mean all those S19 class devices are powered up and hashing today. So that may explain why the mix of devices that prevailed four and five years ago show at such an insignificant level on that chart.