The release of the Nunes Memo and then the Schiff Memo by Congress is enough to defeat the FBI's use of a glomar response on a two page summary of the Steele Dossier. The opinion can be found here.
I really don't think the government had these types of documents in mind when they passed the FOIA half a century ago!
After this was originally posted, someone asked what this means - here's a little background on a "glomar" response - this means a FOIA request was responded with the agency can not confirm nor deny that the requested records exist and if they did exist they would be subject to a FOIA exemption such as Exemption 1, 6 or 7(C). (A recent case I represented the requesters on said Exemption 5 was not appropriate for the glomar response). When a glomar is found inappropriate, the agency will have to confirm whether or not records exist and release them or justify a FOIA exemption to withhold them.