Transpiling is a specific type of compilation that involves two languages with a similar level of abstraction.
Compilation involves languages with different levels of abstraction.
Nim offers a much higher-level abstraction than C, thus you could consider the process of converting Nim to C, a compilation process rather than a transpilation process.
Edit -
Sorry - that was rude. I should have elaborated.
Transpiling is a specific type of compilation that involves two languages with a similar level of abstraction.
Compilation involves languages with different levels of abstraction.
Nim offers a much higher-level abstraction than C, thus you could consider the process of converting Nim to C, a compilation process rather than a transpilation process.