> What is bothering me is the use of "tu" instead of "vous" :
> 
> I don't really know how to react to this. Why being so informal ? Am I
> over thinking this or does it scratch you too ?

You are overthinking this by a huge margin.

But if it bothers you this much, instead of pondering why someone would use "tu" instead of "vous", just ignore this recruiter and only respond to recruiters who are much more formal.