Much of El Salvador's recent infrastructure development is completed by Chinese companies and subsidized by various Chinese government-linked development funds.
They have a massive infrastructure industry with horrendous overcapacity, so they are trying to make train stations, airports, shopping malls and stadiums their primary export
There's definitely the soft power element, but in this case I think it was relatively transactional. "If us Central American economies formerly recognize the People's Republic, what do we get in advance?" And infrastructure was what China had to offer