Not a doctor, haven't read the paper, but had many ASIA assessments before, during and after nerve damage in my spine that was rectified by draining a load of Step B that had formed a little pool party in there (yeah, I know...)
My short take on this, is that even a tiny amount of impingement can have a massive impact on ASIA scores.
The nerves running through your spinal cord are incredibly delicate, and it doesn't take that much to cause them to start going quite wrong.
A relatively small amount of abnormality in my case around T6-T7 area caused me to lose sensation and motor control below the abdomen. You can see before/after MRIs here: https://medium.com/@p7r/one-year-on-2b624075b0bc
Whilst the "object" is some 20mm-30mm long, the impingment into the nerves is perhaps 3mm deep at its maximum, but is pushing the nerves into my spine itself causing two pinch points on the left hand side.
That was all it took to not being able to walk or feel almost anything in my legs, or feet.
My ASIA scores were mostly 0 (no sensation), with some 1 scores (impaired sensation). A month after surgery I was scoring 1 almost everywhere.
Nerve regrowth is magical in that it just works, but it is very slow - around 0.1mm per week is what I was told - and it took me six months to not need a walking stick and have close to normal proprioception below the abdomen. Today - two years later - I still have slight numbness around the ball of my right foot (strangely it's temporal - it definitely has bad days), but am able to walk, drive and do so on without issue.
My ASIA scores now would be 2s everywhere with a single 1 on a specific point of my foot, but even then it's close to being a 2.
So, it is definitely possible that even a tiny, tiny amount of impairment into the nerves would lead to 0 and 1 scores that after 6 months move to a 2, even with pretty minor impingements.