They don't actually solve the problem in 2 seconds - at that point, they are running on a sample of only 3,000 vectors! Then they get it down further, but still find it will take a loooooong time to get through all 3B:

"With these small improvements, we’ve already sped up inference to ~13 seconds for 3 million vectors, which means for 3 billion, it would take 1000x longer, or ~3216 minutes." ...which is about two days.