Nim isn't compiled to C in the same way that, say, Coffeescript is compiled to Javascript.

Nim's compiler converts the AST into an intermediate representation that can be compiled to several backend. The primary backend is C source code, but it also supports outputting Javascript (experimentally) or interpreting the intermediate representation ala Python.

The difficulty of developing the IR -> C transformation certainly influences the features of the language, and certain features, like tail calls, can't be implemented because Nim expresses functions as C functions for the benefit of foreign code. I wouldn't say it's a leaky abstraction though, not any more than C is a leaky abstraction over machine code.