Interesting that they've published this and apparently taken it at face value:
One problem is that unless you know exactly what you're looking at (if it's not between you and something of a known size and a known distance - i.e. in the sky) then you cannot state, from an eye sight-only viewing, what size something is and if you don't know its size then how do you judge the distance?
(Or vice versa- if you KNOW the distance you can estimate the size).
One relatively "famous" viewing (I say relatively because it did the rounds when I was in the Observer Corps and was related to me by the guy on duty in the radar tower at RAF Cottesmore at the time: they were contacted by an incoming RAF flight (trained observers!) to complain about an air-miss (the official terminology when two aircraft are within mid-air collision possibility distance) - the pilot stated, somewhat irately, that ground control hadn't informed him of the C-130 Hercules (40 metres wingspan) at the same altitude within 5 miles and that he'd had to change course to avoid it.
His reputation didn't get enhanced when ground control told him that it was actually a C-5 (68 metres wingspan) at fifteen miles and on a diverging course.
One slight misidentification and the whole situation resolved itself incorrectly.
