Aside from the points others have made that the congestion charge introduced by the mayor of the large urban area of London which covers the whole of central London[1] doesn't really have anything to do with the City of London boundaries or administration, I don't think "it's not well known in America" justifies the use of "secret"

The jurisdictions of San Mateo and Santa Clara aren't particularly well known in the UK but I'd still think any journalist referring to them as "secret" was being ludicrously hyperbolic...

[1]an area with approximately 17x the population of the City of London, and I suspect the ULEZ emissions charge which now covers all 9 million London residents also gets dragged into the US debate...