Interesting. There's no reason to log the North Concord AFS track as more than a coincidence. I put the radar plots on a map and they're about 40 miles from the Indian Head area (as well as several hours early). But it is an interesting coincidence, and the ATIC Electronic Branch explanation is not cogent.
Their reasoning is that the target was "relatively slow", moved "erratically" and also "hovered" at great height, therefore it was probably a "weather balloon".
There are several reasons why this is unlikely.
1) The target was observed for 18 minutes without changing height. Balloons tend to ascend if buoyant, descend if ruptured. A balloon staying at 62,000ft without doing either is in an unstable situation. To stick there for 18min would require a slow leak at a rate that fortuitously preserved neutral buoyancy. This is not at all likely.
2) The target presentation was described as like that of a "large aircraft". No weather balloon payload has a radar cross-section comparable to an aircraft, never mind a "large" aircraft.
3) An "erratic course" is cited as evidence of a balloon. However,
i) the first leg of the track between plots at 196deg, 84mi, 2122Z and 199deg, 78mi, 2126Z is on a NNW heading (~330deg) for about 7.5mi at a speed of about 110mph.
ii) the second leg, still at the same altitude, makes a right turn of about 50 deg onto a heading of ~019 deg for about 2 mi at 60mph
iii) at this point the plot stops and remains apparently stationary, still at the same altitude, for a further 12 minutes before abrupt signal loss.
These motions, on divergent headings at speeds varying by over 100mph within a small area of a few square miles, have no sensible relationship to any likely winds.
I doubt this was a weather balloon.