Perhaps the Balrog underestimated Gandalf, thinking it could cast him into the abyss, then fly back up to deal with the rest of the Fellowship. Alternatively, maybe it calculated that its best chance of defeating the wizard was to grapple with him at close quarters. Either way, the Balrog falling doesn’t necessarily mean that it wasn’t capable of flight.
That isn’t what NATO Restricted means, at least not in the US—I’m not going to attempt to delve into Hungarian law. It falls under the category of Controlled Unclassified Information (CUI) established by Executive Order 13556, and codified in DoD Instruction 5200.48
This information is not freely available to the public, and must be cleared for release according to the procedures detailed in the aforementioned documents, which specify that it should only be disseminated for official government purposes.