Your point would be valid on its face, but I don't think it is relevant unless you could show that the technique in question was "lost" and wasn't even able to be replicated by a society that was capable of transplanting human organs, mapping DNA, and landing men on the moon.
As for how it could be created today ? - perhaps the answer would be to use the paints available at the time, store it in containers made of certain wood and metal which oxidizes, take it out into certain light and humidity conditions, perhaps wash it numerous times with water of different chemical properties in a specific order, hang it up allowing incense to permeate it, subject it to various temperatures from fires .... after hundreds of years of this, it may look exactly like the shroud.
And if you sat a monkey down at a typewriter, he could eventually type a random sequence of letters, spaces, and punctuation in such a way that he replicates the Declaration of Independence.
Your "explanation" is typical of the last resort of a scientific process that is at a loss to explain something: Simply offer an explanation that can't possibly be adequately tested, and leave it at that.