A lack of strength increase would be particularly weird. I thought the subject was weight and body composition. The most dramatic early strength increase comes from the 'neuro' part of 'neuromuscular' so lack of strength increase on a given strength related task when going from sedentary to performing said task regularly would indicate a much more significant problem than merely failing to gain significant muscle mass.

That would imply that he has a neurological disorder that impairs motor function only up to the extent that it prevents performances to improve past the requirements of a sedentary lifestyle, but not to the extent to cause actual disability. Is anything like this documented in medical literature?

There has been enough information provided that we can reasonably hypothesize that Eliezer's exercise response is at least a standard deviation or two in the direction of "genetically disadvantaged" on the relevant scale of exercise response.

It seems to me that Yudkowsky is quite prone to rationalization: he might have started to train, didn't particularly like it and when he didn't get the results he hoped for, instead of revising his training program or keep training for a longer time, he came up with the weird genetic condition as an excuse to quit. At least, this explanation appears to be more likely than the hypothesis that he actually has a weird genetic condition unknown to science, AFAIK.