Still not quite sure what you mean by "as proportionate as everyone else".
Do you say that everyones tibia/femur ratio is the same? (cause that statement doesn't make any sense to me at all)
I am pretty sure what he means is that just as with height, the vast majority of the population fall within a certain proportions ratios range and thus is normally proportionate, even very short or very tall people. Lengthening any bones will quite quickly move where you are on these bell curves and could make you a statistical outlier, that is, getting ratios that would not readily occur in nature unless there's likely some medical abnormality (ie growth disorder or deformity) involved.
I could get back to you with some research papers showing some examples of these kinds of ratio ranges if you want.. at least for sitting height vs height.