The principle of objectivity is a subjective assumption. It can only be determinative if ID is valid. That is, unless ID is assumed, objectivity has no meaning.
Exactly. Rationality has no meaning unless there is some ultimate source of rationality in the universe, which Christians refer to as the Logos (John 1:1).
Thomas Nagel argues this case, against his will, in "The Last Word" (Oxford, 1997). Nagel is a brilliant philosopher, considered one of the dozen most prominent philosophers living in the world today. He is by inclination an atheist, but the problem he frankly confronts in this book is that if you want to have anything like a "strong" version of rationality--something that is really there, not just an illusion, something that allows us to have a common language when talking about the nature of things--then it is very, very difficult to account for it without turning to religion, which he refuses to do.
Thus, if we are all incidental products of a purely materialistic evolution, then we are incapable of true rational thought. Rationality is merely a delusion.
A study of the history of science reveals very clearly that science and technology rose in the West as a byproduct of the rise of Christendom. Lynn Thorndyke has done some excellent work on science in the middle ages, which are far from the dark, superstitious times that people since the Renaissance have depicted. Alfred North Whitehead makes the persuasive argument that science in the Renaissance also is only explicable as a product of Christian assumptions--in particular the assumption that the universe is real and that it is rational.
Modern agnostic scientists are, in a very real sense, still parasitic on these earlier Christian assumptions, of a rational universe and of human free will that can understand things by means of rational thinking and investigation.
Scientifically gathered data, such as fossils and DNA analysis, is objective in that it is tangible and real. Scientists may disagree as to what it means and how to interpret it, but it exists. ID has no means of gathering data that can be measured in the real world. Therefore, all claims of ID are subjective.