Everyone benefits sure, but not "equally well". It variable for most people. The flat out aerobic benefits are great, but for some people it doesn't convert well to sea level performance. Furthermore, the anaerobic efforts at altitude are slightly slower than the necessary pace, and some people don't respond to that as well when it comes to a sea level performance.
Furthermore, Day went up to altitude and also ramped up his training incredibly from 25/35 miles a week to what I'd assume is at least 90 miles per week if not more. So if he's responding well to the aerobic/anaerobic benefits when he comes down to sea level, that makes a big difference on top of his mileage benefits.
For example, I did a high school training camp that was about a month at altitude, and after a week I was able to keep doing easy runs around 7:15 pace, which was the pace I typically ran for easy days beforehand. But when coming back down to sea level, my easy and steady paces didn't change, but my anaerobic level improved like crazy to where I could start doing my 400m repeats about 3 seconds faster comfortably (mind you I was running 400m repeats at 800m pace).