US Company with Puerto Rican employee

[Gilmore]

I have read some of the previous discussions on how to treat an employee of a US company with a US 401(k) plan that is a resident of Puerto Rico. 

I think I understand that if the employee is a bona-fide resident of Puerto Rico, and all service for the US company is performed in Puerto Rico, then the compensation earned is considered to be non-US source income?

If that is the case, and the plan document does not have dual provisions for US and Puerto Rico, is the employee treated as excludable for coverage, or are they non-excludable, non-benefitting?

Thank you very much.