The following is taken straight from
NWT Study Notes on Colossians 2:9:
it is in him that all the fullness of the divine quality dwells bodily: The context shows that having this “divine quality” does not make Jesus Christ equal to God Almighty, as some claim. In the preceding chapter, Paul states: “God was pleased to have all fullness to dwell in him,” that is, in Christ. (Col 1:19) So the Father is the one who caused Christ to have “the fullness of the divine quality.” At Col 1:15, Paul says that Jesus “is the image of the invisible God,” not God himself. Col 1:19-22 describes the reconciliation that God brings about through Christ, and Col 2:12 shows that God raised him from the dead. Furthermore, Paul later says that “Christ is seated at the right hand of God.” (Col 3:1) These statements show that possession of this “fullness” does not make Jesus Christ identical with God, the Almighty.
of the divine quality: Or “of the divinity.” The “divine quality” includes all the excelling qualities of Jesus’ heavenly Father and God, and these also dwell in Christ. The Greek word (the·oʹtes), which occurs only here in the Christian Greek Scriptures, is derived from the Greek word for “god,” the·osʹ, but is different in meaning. Many lexicons give such definitions as “divine character; divine nature; divinity.” The term was used by ancient Greek writers to describe a quality or condition that could be obtained or lost as a result of one’s behavior. Obviously, then, such a term was applied to created beings and not exclusively to the almighty and eternal God, Jehovah. So there is solid basis for rendering the·oʹtes to refer to a divine quality rather than to God himself.
Jehovah‘s Witnesses believe that Jesus as the Word (John 1:1-3) or Wisdom (Proverbs 8) was only begotten (John 1:18) or brought forth/born (Proverbs 8:22). While this could be called a creative act by God Almighty, it differs from all (other) creation, in that everything else was created by GOD through the Word/Wisdom/Jesus as God‘s „master workman“ (Proverbs 8:30; Colossians 1:15,16).
It could be argued that God who begat Jesus in the beginning, does not consider Jesus as part of creation, but as his only-begotten Son/God, which is why Jesus is excluded from the created things mentioned in John 1:3 and Colossians 1:16. Jehovah’s Witnesses in this context add „other“ in between „all things“ as Jesus is called „the firstborn of all creation“ in Colossians 1:15. This is not impossible, for Paul clarified a similar wording in 1. Corinthians 15:27 when he says: „For he [God] has subjected everything under his [Jesus‘] feet; but, when it says ‚everything‘ is subjected, evidently exclusive of him who subjected everything to him.“
Being God Almighty‘s only begotten Son, means that Jesus is the only person brought forth (or created) by God Almighty alone. „Begotten“ conveys the meaning of „reproduction“, such as is made clearly evident in Matthew 1:2-16 (KJV) where „begat“ is used multiple times in the sense of „human reproduction“.
Now God Almighty did not have a partner to mix genes, which could be interpreted that he produced a copy of Himself resulting in Jesus. This is a biblically acceptable interpretation as Jesus is called „the perfect image“ of God, the „perfect representation of [God‘s] being“. When a perfect Almighty God reproduces Himself, wouldn‘t you expect an exact and perfect copy/representation as a result?
Wouldn’t you expect „all the fullness of deity“ to dwell in that perfect representation?