1. Passing off only applies to unregistered trademarks. For registered trademarks, it's just trademark infringement. Since ubuntu is a registered mark (or so they mention in the C&D), passing off simply would not apply.
2. Nominative fair use, as I mentioned, is a complete defense to trademark infringement (including passing off), and most other related things, including dilution.
If FixUbuntu claimed or implied they had a business relationship/etc, there would be an issue.
They don't.
Trademarks do not give you the ability to stop others from using certain words, they give you the ability to stop others from confusing consumers about the source of goods.
Plus, you know, criticism of products is among the most protected speech possible when it comes to trademarks ...
2. Nominative fair use, as I mentioned, is a complete defense to trademark infringement (including passing off), and most other related things, including dilution.
If FixUbuntu claimed or implied they had a business relationship/etc, there would be an issue. They don't.
Trademarks do not give you the ability to stop others from using certain words, they give you the ability to stop others from confusing consumers about the source of goods.
Plus, you know, criticism of products is among the most protected speech possible when it comes to trademarks ...