He said miners will mine more at the same expense, not just "more." That is true if difficulty were to drop. Individual miners will use the same electricity to produce more coins for the same cost in power.
Miners will not "mine more at the same expense". They will mine the same thing at less expense.
Individual miners cannot "produce more coins for the same cost of power" if not other miners "produce less coins for much less power".
Because the total number of coins mined must be the same.
If some use "the same amount of power" and hence "mine more", then that means that other miners mine less, and consume vastly less power (maybe stop mining all together).
Globally speaking, the miners mine the same amount of coins, and use less electricity. The do not mine more coins with the same consumption. Some do, and others mine less.
That means that miners make more profits (or lose less money, depending where they come from).
The point is that it could be that those increased profits go into hodling coins. That's the ONLY way in which difficulty can have an influence on the price: by increasing the demand for holding coins (by the miners themselves, because they want to allocate their profit in coins).
If difficulty goes down then a miner will mine more coins with the same machine. The same machine will obviously continue to use the same amount of power. Not sure where you are getting your information but it is inaccurate.
The total number of coins mined remains essentially unchanged if difficulty goes down.
If, as miner A, you keep your machine running at the same power, you will of course mine more coins.
But that has to be compensated by another miner B who is mining LESS coins for the sum to remain the same, right ?
Now, with difficulty going down, that means that in order for miner B to mine less coins, he has to reduce power consumption, and more than proportional to the reduction of the amount of coins mined (as difficulty went down), right.
So yes, some miners, like miner A, will mine more coins at same power, but other miners, like miner B, will mine less coins at much less power.
So on average, your miners are going to mine the same amount of coins, and use less power. That is exactly what difficulty expresses !
If all miners were to use the same power and mine more coins, difficulty would not go down.
The very fact that difficulty goes down means that power consumption (at equal technology) goes down. As the total number of coins mined is a given, it means that a miner, on average, spends less power and mines the same number of coins. Of course, some will mine more and other will mine less.