I don't think that's fair. Sure HH has been going on for eight years, but on average Casey is only putting in 3-4 hours per week. That's not even half a work-day per week. Not to mention that he spends a lot of time explaining every single decision he makes and does black-boarded lectures on pretty much every subject covered in the series.

I think it's reasonable to assume progress would be at least 3-4 times faster if his focus was solely on the code. So what you call "a billion years" is actually more like a total 2-4 months of fulltime coding.