The remark is right as to the ambiguity of the English, because 'come' is also the participle 'have come,' and the natural connection is, "sinned and come short." But it seems to me that ὑστεροῦνται does not refer to exhibiting. With a genitive, and particularly in later writers, it has the sense ‘destitute of,' ‘wanting,’ ‘failing to have.’ Now that sin has come in, we must meet the glory of God or be excluded by it. In a state of innocency man enjoyed favor, and the question of consistency with the divine glory had not been raised. Now, we say, "All have sinned, and do come short of [fail in meeting, or standing in the presence of] the glory of God." Christ, as Son of man, has glorified God on the cross, and human nature has a place in the glory; οὐκ ὑστεροῦται, and so we in Him. This point of meeting the glory I believe to be an important one, and to run through the gospels. John 13 specially treats it with immense depth, though briefly. I add that ἥμαρτον, the aorist, is the historic fact, which is the ground of the present state expressed in ὑστεροῦνται. We have sinned, and are outside of, away from, morally wanting in what meets and gives us a place in, the glory of God.