Bro i linked the thread like 2 posts down. Check it out. He built a fucking proper LED strip build. He could have got 900 watts using like 6 double row strips, which wouldn't be the most uniform light in the world. But he invested WAY more cash into the light and used 36 strips to get near perfect uniformity. Most people cant run 40w sq foot of LEDs of this efficiency without significant stress/bleaching, or use of CO2. And that's because they run say 4x cobs at 150w each over a 4x4 tent, and the hotspots are no joke.
When your light is that uniform, evenly spread, and diffuse, you'll flat out have healthier plants that yield more. Because there's no hotspots causing stress, or parts of the plants getting totally different amounts of photons.. It's even been proven in several studies that having light that's more diffuse/uniform flat out boosts photosythesis, yields, crop quality, plant health, etc.
So it's honestly not that crazy that he managed to yield that well considering his setup.
But of course some HPS lovers have to be in disbelief that a guy can pull those numbers his second grow. You can doubt what you want to, but when you arent growing under a space heater that emits a spectrum that is so unnatural that it distorts the leaf sizes, internodal distance, and bud structure (this elongation means the cell structures are weaker too), yeah it might take you 5 grows to even manage anywhere near a gram per watt. But when the light is working in your favor, you just have to read the plants and grow the right strains and you will get good yields period. All the decent hps growers use crazy ass techniques that are often a bit rediculous just to utilize their inefficient HPS light.