Notice two important things. First, he doesn't say, "You alone are the true God," (i.e. and therefore I am not) but "You are the only true God." Since, "the only true God" is synonymous with "God," are we to believe that Christ's affirmation of the Father's being God is a denial of His own divinity? Or worse: that Christ is a false God? Rather, He constantly affirms He can do nothing without the Father: He is not a renegade or other god, nor asking you to believe in another: "You believe in God [i.e. the Father], believe also in Me" (Jn. 14:1). He is the expression and image of the invisible God itself. No one has ever interacted with God except by means of His Word. The Son is God for the same reason the Father is God: they belong to the divine nature fundamentally, and Scripture affirms this in multiple places (e.g. Jn. 1:1; Phil. 2:6; Rev. 1:17 cf. 22:13).
But inasmuch as Christ has two natures, including a real human nature, making Him a man as well as God (not a God which is a man, Num. 23:19), He can say things not true of His divine nature but true of His human, and vice versa, or speak of Himself as separate from God or the Divinity, since His humanity truly is distinct and different than the Divine Nature: His person only uniting the two (i.e. the Hypostatic Union, or the fact that Christ's two natures cohere only in that the same Person owns them both, not that there is confusion or admixture between them).
For example, He can say, "I am the First and the Last," and in the same sentence, "I died" (Revelation 1:17). Or St. Paul could write, "they would not then have killed the Lord of Glory" (1 Cor. 2:8). Clearly God cannot die, but the sense in which God took on a human nature allows Him to say He died in a sense which doesn't violate this fact: and which moreover is a very plain and obvious sense.
In other words, if the Father is not the only true God, then the Trinity is false, so it cannot possibly be an argument against it. Likewise, if Jesus can't distinguish between "Jesus Christ whom you have sent," and the Divinity, then the Divinity and the incarnate Jesus are identical, whereas this would be the heresy of Modalism, Monophysitism, and Patripassionism all mixed into one monster heresy.