Costa Rica and Paraguay coverage was added recently (within the last year iirc). The author notes Paraguay as an example of a country that was not yet in the dataset they sourced from.

El Salvador does have a decent amount of coverage on street view, but this was done by El Salvador Maps (if you pan the camera down, you'll see this name on the cars used to capture the coverage). The dataset is curated by a member of the Geoguessr community, in which "unofficial" coverage like this is disregarded, which is why you won't see it included.