Understanding namespaces
Straight from the PHP Global space docs:


  Without any namespace definition, all class and function definitions are placed into the global space - as it was in PHP before namespaces were supported. Prefixing a name with \ will specify that the name is required from the global space even in the context of the namespace.


Devel's dpm() is declared in 'Global space', and since there's no other dpm() function declared in your namespace, the 'global' one is used (no need for the preceeding \).