In general he makes assumptions like asic X takes Y material and runs Z years and then you throw the hardware away. Divided by all transactions it makes up the ewaste of 2.7 iphones per transaction.
The amount of transactions doesn't matter when mining blocks. Now with rising adoption or even with monkey jpegs, there might be more transactions, but same "e-waste", so less iphones "per transaction".
Asics aren't thrown away unless they are really broken, they get cheaper and someone with excess energy buys and runs them years after his assumed end of life. Buy a used S9 and run it with solar in your home or use it as heater, it is still in his assumptions as being waste.
It's more of a theoretical construct than actual real world statistics.