Sorry but but that is complete nonsense and if interpreted like you are interpreting it, you have John contradicting his own teacher in John 17:3 where Jesus made it clear that the Father as The Only True God and himself as the Christ whom he sent" is the eternal life of the believer, for that is actually what 1 John 5:20 says as well.
Here let's examine it again.
1 John 5:20 And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.
Notice first, John is making a clear distinction from the true God and his his Son who came to give us an understanding of the true God and in order that we might now him who is true and which is obviously the Father.
Then what he is saying after this, is that we are in the True God by being in his Son and again we see this distinction between the one being called The True God and his Son and John never reveals the Son here as also be the true God, never!
So when he finished by saying "This is the True God and eternal life" he simply means that it is both the True God (The Father) and Jesus his Son whom he sent that is the eternal life of the believer and just like Jesus who taught John says in John 17:3 also.
Sorry, but in how you are twisting 1 John 5:20 to be meaning, you have John who was the disciples of Jesus and taught by Jesus, contradicting Jesus' very clear words in John 17:3 and which reveals that your doctrine is false also.
What is really amazing to me, is how trins will go to college to learn Greek and Hebrew grammars but then when they read the clear and simple grammar of Jesus in John 17:3 and also that of 1 John 5:20, they totally reject it for their preconceived personal bias that Jesus is God and that God is a trinity.
This is why I wouldn't trust their academic knowledge if my life depended upon it, for they can't even be honest about what Jesus clearly said in John 17:3 at all neither can they be honest with 1 John 5:20 either, for neither passage includes Jesus as being the true God but both reveal that it is the True God and his Son who he sent alone, through which the believer has eternal life.