"A seasoned C developer with 10 years experience can not instantly morph into an advanced front end web application developer"
True, but they'll be a damn sight quicker at picking it up than anyone else. And they'll write far better code when they're there.
If you're looking for a permanent employee, all development experience is relevant -- there is a huge amount of overlap and retraining just for a different language is no biggie. They'll also have solved (i'd bet) far harder problems than "HOW DO I GET THIS UI WIDGET TO UPDATE WHEN THIS HAPPENS".
If you're looking for someone to hit the ground running from day 1, or work as a contractor, it's different. But you generalise too far here.
"However, they have failed to mention anything else to back it up. I would expect to see some of the following throughout their CV; Design Patterns (MVC, Factories, Singletons, ActiveRecord), Classes, Objects, UML…"
In my CV (highly rated by everyone i've given it to) you won't find a single one of these. Instead, you'll find real descriptions of what i've accomplished, who I worked with (mentoring etc), my methodology (agile, test driven development) and so on.
And UML... holy shit, yeah, no.
Community work? See Ashe Dryden's "The Ethics of Unpaid Labor and the OSS community", http://www.ashedryden.com/blog/the-ethics-of-unpaid-labor-an..., and James Coglan's "Why GitHub is not your CV", https://blog.jcoglan.com/2013/11/15/why-github-is-not-your-c....
Hiring the right people has always been, and will continue to be, a hard job. The only way to do it is to use common sense and actually read the CV's you're being sent, then talk to the people you think could do the job to see if they can.