You'll find a newer guide here: http://wiki.xbmc.org/index.php?title=Installing_XBMC_for_Linux#Ubuntu
You'll notice it's even shorter than the Windows one. :)
Quite why there are two versions of the same guide is a good question, but that is not the fault of GNU/Linux; poor documentation is a general problem with all software products. Did you offer to update the section of the guide that caused you problems (assuming you solved them) to give back to the community and help others? Or at least point out where you came unstuck so that someone else could?
Personally I prefer the extra step of adding a PPA/repo before install. The security (and that'll be cryptographic signing) that is offered educes the chances of getting pwned. Tell me, do you check the md5hashes after doing a download, to make sure you've got a legit file?