It wouldn't make sense for them to not open lightning channels to other entities outside El Salvador. If El Salvador will never spend their Bitcoin, they might as well not have it.

A Fedimint would be a good idea within the country but I hope the people will be able to easily and directly swap their ecash for ecash in other federations and/or make outbound lightning payments. El Salvador could take fees for swaps and lightning sends while still working hard to offer a place for those who would like to be financially sovereign individuals.