This has nothing to do with USD price of bitcoin...
I paid 300 BTC for them. Yes they were sold in USD but even if BTC were $1 each, I will most likely not be able to mine the 300BTC I paid for them back in a reasonable amount of time.
Of course not, but you will be able to mine back the cost of the unit in $USD in a reasonable time, and that's the point. The unit was sold to you priced in $USD not BTC, whatever you choose to sell to get sufficient $USD at the time of the order is your problem. Personally I would have used something that inflates over time, like a fiat currency to pay for an item with a long lead time.
I believe that he means he sold btc to buy the miner. So if he wouldnt have done this and kept the BTC he would have way more $USD now than this miner can mine ever. I can understand that this is a pain.
Of course miner sellers need people that think that their miner, once bought, will be pay back faster only because the bitcoin exchange price will go up. Good for the seller then.