No, God being Holy Trinity is not merely His "fullness" but it's of His essence (loosely speaking, since God just has existence of course). Our Lord taught quite clearly that "No man cometh to the Father but by Me." (John 14:6)
You can say that there's a God, but unless you worship the objectively True God, as He has revealed Himself, you're not worshipping God as He is but rather your concept of God, which could be a figment of your imagination.
This Vatican II speak about "fullness" (cf. my criticism of +Fellay's 95% comment) has done tremendous damage.
Let's take this analogy. I'm in love with some celebrity, say, some movie star. But this love is based on my conception of her based on the characters I've seen her play in the movies. But when I actually get to know the REAL person, as she is, I find that she's insufferable. And she's not even particularly attractive once she loses the movie makeup. So, did I really love this celebrity? No. I loved some figment of my imagination and some fictional portrayal. Same thing with God. They worship their CONCEPT of God, which is deeply flawed, but they do not worship the True God, as He is, as He has revealed Himself to us.