It isn't. It doesn't matter if he has every GPU in the entire world starting at different private keys and adding one each time as he proposes to try to find a specific public key. Even if he had 1 trillion GPUs and each one could check 1 trillion private keys a second with no duplicates, and he had done this every second since the birth of the universe 13.7 billion years ago, he would only have covered somewhere in the region of 0.0000000000000000000000000000000004% of all possible private keys.
But he's talking about renting over 1000 GPUs. surely something good would come of that. that's alot of horsepower. have you actually tried what you're talking about because otherwise I'm not sure how you could know for sure.
also this doesn't have anything to do wtih the OP but I did find it noteworthy that there are apparently 2 exascale computers now in china. I bet if they threw those things into brute forcing bitcoin private keys they would get some pretty fast.
Brute force 256 bit key using 1000 GPUs...longer than the universe * universe
Kangaroo a 256 bit key using only 1000 GPUs...longer than the universe
You do not have to try renting 1000 GPUs to get an estimate on run time.
256 bit = 255 / 2 + 1 = 2^128.5 group ops to solve key
Now take 1000 GPUs and let's say their speed is 2,000 MKey/s: 1000 * 2,000,000,000 = 2^40.8
so 2^128.5 / 2 ^40.8 = 2^87.7 seconds to solve.