What is that metal clip on this video?
 

Fast forward to around 3:43, the guy unclips the metal retainer at the back of air intake. Well, I am curious if this is model specific because my 2008 C300 doesn't have it and I've been getting P1070 and P1073 ever since I got this car last year.

Not all M272/M273 engines have this clip. It really does not do much. Even if you left it off, it should not cause any issues as long as you have installed the filter housing properly.