I find this argument strange, especially since he uses the word "seem". "Seem" does not mean "is".
If you really want to make life seem short, measure it in millennia. You only get a fraction of one.
Personally, I feel it's about your state of mind. If you're depressed, life feels unbelievably long. If things are going well, then you don't want the good times to ever end, and it seems short.
I don't really know how to conclude whether life is or isn't short. All I can conclude from the article is that PG is quite happy with his!