A. H. Strong’s comment that you quoted above is very unfortunate, and of course there are also others who do not understand the difference between an outward, physical act and disobedience to God. The physical body is absolutely incapable of disobeying God, and hence it is absolutely incapable of sinning. The sin is always and exclusively in the mind, never in the members of the body.
Let’s consider for a moment the Sixth Commandment, Exod. 20:13. You shall not murder. (NASB, 1995) The word translated here ‘murder’ is the Hebrew word רצח, and although it is not the usual word for taking a life in the Old Testament, it does not imply intentionality as can be seen by its use in Deut. 4:41f, Josh. 20:3, etc. where one kills unintentionally. Quite obviously the Sixth Commandment does not forbid the physical act of taking a human life unintentionally, and yet the physical act of taking a human life is exactly the same whether it is intentional or unintentional. The sin of murder is only in the heart; it is not in the physical act.
Staying with the subject of the murder, consider the following from the first chapter of Romans,
28. And just as they did not see fit to acknowledge God any longer, God gave them over to a depraved mind, to do those things which are not proper,
29. being filled with all unrighteousness, wickedness, greed, evil; full of envy, murder, strife, deceit, malice;
they are gossips,
30. slanderers, haters of God, insolent, arrogant, boastful, inventors of evil, disobedient to parents,
31. without understanding, untrustworthy, unloving, unmerciful;
32. and although they know the ordinance of God, that those who practice such things are worthy of death, they not only do the same, but also give hearty approval to those who practice them. (NASB, 1995)
In this passage the focus is on murder in the mind (“being filled with all unrighteousness . . . murder . . . .”), and as verse 32 so very clearly says, that murder is just as much murder as if it is carried out by oneself or found only in ones consent to the physical act of another.
And, of course, we have testimony of Jesus in the Sermon on the Mount,
Matt. 5:21. "You have heard that the ancients were told, 'YOU SHALL NOT COMMIT MURDER' and 'Whoever commits murder shall be liable to the court.'
22. "But I say to you that everyone who is angry with his brother shall be guilty before the court; and whoever says to his brother, 'You good-for-nothing,' shall be guilty before the supreme court; and whoever says, 'You fool,' shall be guilty
enough to go into the fiery hell.
Whether or not the physical act was carried out is incidental to the fact that a murder has taken place. (Of course it is not incidental to the family and friends of the victim, but that is a whole different matter). The physical act could not have been the sin, because there was no physical act in Matt. 5:22.
And we find the same sort of teaching regarding adultery just a few verses later,
27. "You have heard that it was said, 'YOU SHALL NOT COMMIT ADULTERY';
28. but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart.
The words of Jesus here could not be more explicit—the sin of adultery was committed, NOT physically, but “in his heart.” If the physical act was also the sin of adultery, every time a man or woman commits the act of adultery they would be committing adultery twice—once in their heart and once in the physical act. Quite obviously A. H. Strong had not adequately thought this through, and, as you pointed out, he is not alone in his error.