Forbes is reporting[0] that they are using BitGo. FYI Salvadorians are not required to use chivo at all. The interoperability is the exciting part not chivo.
[0] https://www.forbes.com/sites/jonathanponciano/2021/09/07/el-...
[0] https://www.forbes.com/sites/jonathanponciano/2021/09/07/el-...