I don't think you can conclude this without comparing the blocks before lower fees were accepted. If, for instance, 6 million transactions paying 1 sat/vbyte were replaced by 9 million transactions paying 0.3 sat/vbyte, miners earned less.
I don't think he did that. I understand what you are saying, though. I think he only considered the difference between what the miners earned, mining blocks with transactions paying sub 1 sat/vByte fees, compared to what would have been the case if they didn't. Let's not forget, mempools were quite empty for a time, which is also why miners lowered their standard relay fee. If there were enough +1 sat/vByte transactions to be mined to fill blocks, miners wouldn't have made any changes, and there wouldn't be space in blocks for sub 1 sat/vByte transactions.