The commentary I'm reading agrees with Aquila without clarifying much specifically, but argues that spiritual death as a meaning here is doubtful
http://biblecommenter.com/1_john/5-16.htm
A sin which is not unto death - The great question in the interpretation of the whole passage is, what is meant by the "sin unto death." The Greek (ἁμαρτία πρὸς θάνατον hamartia pros thanaton) would mean properly a sin which "tends" to death; which would "terminate" in death; of which death was the penalty, or would be the result, unless it were arrested; a sin which, if it had its own course, would terminate thus, as we should speak of a disease "unto death."