I agree — but I didn't ask Clarke to give two possible explanations for the photo; I simply asked him what his thoughts were. And like you, I think the hoax theory is clearly the most plausible explanation, for many reasons.Now, if you combined all sorts of hoaxes and hoaxsters into one pile, and THEN asked for a second choice of possible explanations, I guess "secret military test of something" would be a good candidate -- perhaps a drone balloon/dirigible or something. The objections to that are formidable, so I'd not think it likely at all, hoax remains the clear favorite, to my mind. But "secret military thing that didn't pan out or surface in the intervening years" seems much more likely than, say, aliens from Omicron Perseii 8.
That's why I find it particularly interesting that even Clarke considers a hoax to be a likely possibility. It's notable because that doesn't always come across in his interviews or blog posts. By focusing on 35-year-old recollections and highlighting Robinson's analysis that the photo most probably depicts something large in the sky, the overall impression many people get is that the hoax explanation has been "ruled out".
I'm not suggesting Clarke is being deliberately misleading — far from it. I actually find him honest and sincere, unlike many others in the UFO field. But I do think the way he presents the case could give the impression that the hoax explanation has been dismissed. And even though Clarke is an honest investigator, his presentation style may unintentionally support the narrative pushed by more dubious ET proponents.
As for the possibility that the photo shows some secret military craft or experimental equipment — well, yes, that's more likely than it being an alien spaceship, simply because we know the military exists, whereas we have no solid reason to believe Earth is being visited by extraterrestrials. But aside from that, both theories are just as unlikely. (The object doesn't resemble any known military aircraft, and the eyewitness testimony describes behavior — hovering silently for several minutes, then shooting off at high speed — that doesn't match any known human-made craft. Etc etc…)
In other words, if we assume it's a black project, we still have to assume the photographer lied about what he saw. (Let's say he took a photo of a military balloon and made up a story of it behaving like a "typical ufo".) But if he's lying, why trust any part of the story — especially when the photo itself could have been faked using simple materials and basic techniques?
To me, the "secret military craft" theory seems like a way to keep the story alive without sounding like a tinfoil-hat believer.