1) The total amount of time is not the same if you just count GPU-hours. If you have 16 GPUs, it makes sense to run them for 4.5 hours to get to 72h for an even comparison, not 8.
2) If we stop at 4.5 hours(and are generous including the big drop), the loss is about 0.978, which is the same as about 44 hours with the sequential solution, making the sequential solution about twice as efficient.
So the real conclusion here is that we are able to run things in parallel at an efficiency loss but at a time win as long as we have access to more hardware. I feel like the blog oversells itself.