As @googlebot mentions, this initialization is universal, invoked in the constructor of any linear layer. The nonlinearity argument passed to the kaiming_uniform_ function is not based on the nonlinearity that follows the Linear layer. The whole idea is to use the kaiming_uniform_ function to the get the uniform initialization we want, by passing the right set of arguments (a and nonlinearity).
You can maybe think about this as PyTorch devs trying to avoid writing any code that duplicates statements in kaiming_uniform_.