hi, i think you are completely right. if the biceps is to short, you are unable to straighten the elbow, because biceps has the lengthen and triceps to shorten (contract if movement is active).
maybe the article ment if triceps is to weak? check it out again.
it can also be a capsular problem, after immob. the capsule tends to "shorten" and to fix the elbow. manual therapy might help.

good luck