I was trying to find when this issue arose. The Evening Standard date wasn't particularly helpful, but this one may be more accurate:
Kate refuses lie test Sunday Express (no longer online)
Camilla Tominey
Sunday November 18,2007
They were therefore still arguidos, but they obviously wanted to fight back against all the tabloid media smears. However, in those circumstances, I'd find it highly unlikely that they'd have spontaneously offered to do a polygraph test.
What seems more likely to me is that a newspaper asked them if they'd be willing to do one, and found this chap to do it.
It was never going to be more than more sensationalist tabloid fodder that could have been twisted whatever the results.
It might appear to be a natural reaction for the innocent to want to declare that they'd be willing to do one to clear their names, but not when you've been hounded for months on end, with everyone twisting anything you do say, with no guarantees about reliability, nor even the questions that they would be allowed to answer...
And I think that they were right to be wary - the story was no longer that they had passed or failed the test, but that they refused to take one. The tabloids got their story anyway.