Christof was on target when he said that there is no more truth in the real world than there is in Seahaven, - if and only if you believe that truth is relative and subjective. In that case, had they surrounded Truman with Muppets all his life and made him believe that the entire planet was populated by Muppets except for Truman because Truman was an alien wizard; then that would be the truth too. I can't accept that.
On the other hand, if you believe otherwise, then Christof was wrong and was just desperately trying to keep his show in business and his star/slave from quitting. It is worth noting that his argument for there being no more truth outside than inside was: "Same lies. Same deceit. But in my world, you have nothing to fear." That seems dubious and weak, so even if the statement had been true (and Christof really believed it), you could argue that Christof was not right since his belief was not justified (a.k.a. he believed it for the wrong reasons).
Completely unrelated to Christof, however, there would still be people who believe that that the truth inside is true inside and the truth outside is true outside; and that both are valid truths. I can't believe that the Muppets could be a truth, and by extension I can't believe that there can be multiple truths. Similarly, I can only think of two famous proponents of this, like Gandhi:
"It is not given to man to know the whole Truth. His duty lies in living up to the truth as he sees it, and in doing so, to resort to the purest means, i.e., to non-violence.
God alone knows absolute truth. Therefore, I have often said, Truth is God. It follows that man, a finite being, cannot know absolute truth.
Nobody in this world possesses absolute truth. This is God's attribute alone. Relative truth is all we know. Therefore, we can only follow the truth as we see it. Such pursuit of truth cannot lead anyone astray."
"What is true for you is true for you, and what is true for me is true for me."
Though I can't help thinking that they would hesitate to defend the Muppets argument.