^Nick actually changed the date and post dated that BitGold article to make it look like it was posted in 2008 a couple of months after the white paper. The url still says 2005 as you can see in the link you posted. This is also confirmed in emails from Hal Finney.
To me, that's not very good evidence. Why would he post-date it solely to throw people off if you can see the real date in the URL?
He explains why he post dates it in another blog. He basically makes the "Bitgold part 2" blog in December of 2008 and feels like it needs part 1 next to it so that they can be read together. He goes on to explain about miners, miners having to solve problems, and stamping. The blog is called Bit gold markets. http://unenumerated.blogspot.kr/2008_12_01_archive.html
This post is a month after the white paper was released. One might assume Nick read the white paper and maybe updated this blog to reflect the ideas. More likely in my opinion, Nick wrote the Bitcoin white paper under the "Nakamoto" alias and released it to see what others would think, but it wasn't gaining enough traction. So he then wrote about it in more detail on his blog which was getting a lot more hits and attention. At the time it seemed okay because he didn't really know how big this deal would become.
Also, Nick is qouted as having aliases at the time, and it is quite possible that the name "Nick Szabo" is an alias itself.
Seems like to me he came up with the idea in 2005, started coding in 2007, and released it in late 2008.