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.
And they completely missed the mark a lot. It's amazing to me that reporters are considered great if they equal the proverbial broken clock. Printing one truth does not make up for ten lies.
"Deep Throat" may have been fictitious composite figure used by Woodward and Bernstein as a cover for every leak that they acquired from every source.