Naaman's words to Elisha are actually an expression of faith. He is acknowledging Elisha's God as the true God, and admitting that he will no longer be truly worshipping in the house of Rimmon anymore. Because, as a soldier who must follow the orders of his superiors, he does not feel free to openly oppose them, Naaman realizes he will still be called upon to go into the house of Rimmon and to worship there with his master. In his heart, he can no longer do so; yet he feels he must still enter there--and it is for this that he asks forgiveness.
God meets people where they are, and Naaman is still ignorant of many things.
In the New Testament, this specific form of worship is addressed as an act of ignorance.
Forasmuch then as we are the offspring of God, we ought not to think
that the Godhead is like unto gold, or silver, or stone, graven by art
and man's device. And the times of this ignorance God winked at; but
now commandeth all men every where to repent: (Acts 17:29-30, KJV)
In other words, God does not hold people accountable for that which they did not know.
Elisha realizes that it would be neither prudent nor feasible to compel Naaman to stay at that moment for some extended instruction in the things of God. He accepts as Naaman's best the sincere confession of faith he has just made. And God also accepts it.
Jesus, even with his own disciples, was unable to share all points of truth which they should know.
I have yet many things to say unto you, but ye cannot bear them now.
(John 16:12, KJV)
We are often not able to bear all of the truth at once. Paul faced the same thing with the church in Corinth.
I have fed you with milk, and not with meat: for hitherto ye were not
able to bear it, neither yet now are ye able. (1 Corinthians 3:2, KJV)
Naaman was certainly not doing right to bow before the god Rimmon, even though he was no longer worshipping Rimmon. But Naaman's heart was in the right place and he was still ignorant of his error in this regard; therefore, God accepted him as he was, and winked at his ignorance.