This method protects against a drop in BTC value because the hardware itself serves to hedge against the initial investment. If I spend $1500 to purchase 15 Btc and the price per btc declines to $70, then I'm down $450 (I.e. 30 x 15 = 450). But, if I purchase the miner instead and the price drops to $70, I always retain the option to sell the miner. For example, selling the miner purchased originally at $1500 for $1000 at a $70 conversion rate, I can receive 14.28 btc for it, and that's on top of the few btc I would've mined already.
In other words, purchasing a miner with fiat is better if the price of btc drops, but it's also better than doing nothing at all if the price of btc goes up. Buying btc outright may be better if you assume that btc value will only continue to increase, but this is a riskier approach.
You are making the assumption that you can sell the block eruptors for $1000. The argument that the miners retain value in case BTC/USD drops only applies to GPUs. The resale value of an asic is purely determined by how much BTC it can gain. If BTC price drops, the value of your asic hardware drops as well.