I do understand why not all spurce material can be published, but that source material is usually strictly vetted before publication. And that is were the brand of newspapers comes in.
If they aren't willing to publish the files (though many, many sensitive documents have been published while protecting the source!) than they can use a second or third news source to validate the files. This happened with the original panama papers before many of them were publically published - 4-5 papers all vetted them in chunks.
"Trust me bro" doesn't cut it, especially not for The Guardian.