According to
this page, AT&T is using Claro and Movistar as roaming partners.
Coverage is not nationwide. And the statement about Calro is confusing - "The coverage is just in the San Jose Metropolitan area"?
And apparently you can only call Costa Rica and the US - "You may not be able to place calls to international destinations other than the United States while roaming in this destination. Calls can be completed within the visited destination and back to the United States."
Looking at the prices, I'm still failing to see why getting an inexpensive unlocked GSM phone and a Kolbi SIM isn't a better option.
My Project Fi plan works everywhere the AT&T plan works - at no additional cost for data and SMS and 1/5th of the voice cost, but it doesn't work everywhere in Costa Rica - plus it lets me keep my US phone sanitized.