>>In the validating case, the validator is purchasing coin which simply causes prices to adjust and the economy keeps producing the exact same goods and services as if the validation activity did not exist (after a short dislocation, which would be very short in this case because few contracts are denominated in cryptocurrency).
The value diverted to coin purchases ultimately has a cost in goods and services. It diverts economic activity to non-economically productive activity in cycling capital into and out of deposits, which results in less liquidity.
I think Proof of Stake could potentially be better than Proof of Work, but the point about cost being equal across validation methods is correct in general in my opinion. There are specific circumstances where it is not true, like if producing the mining resource creates negative externalities.
Where I think the article is wrong is in neglecting other aspects of consensus algorithm efficacy, like the potential security benefits from Proof of Stake totally aligning the incentives of owners of mining capital (which in the case of PoS is the network coins) with the success of the network.