Not one person, but at least three people "important" enough: Woodward, Berstein and Bradlee.
Guess I'm being a little stupid here but how does this protect them? You're implying there was no deep throat? They made it all up?
They hit too close to home several times not to have an inside source. The only rationale I can see for assigning one person as deep throat was that it made it more dramatically effective. If that's the reason they said there's only one deep throat it won't necessarily tarnish their image.