If it is so accepted and so immaterial then why not just tell the world? I know snowman lists a lot of cards for sale. He also openly, to his credit, admits here to cleaning and improving cards. Not sure where he actually draws the line. Anyway, when I look at his ebay listings oddly I never see him disclose any work done...I guess those cards he has listed are not the ones he has worked on.
And I do think with each passing day fewer collectors care what has been done to the card. I imagine a majority of the collectors who read a disclosure that a card was worked on would be discouraged from buying. They see it passed grading so the assumption is that whatever was done must not have been considered improper. The concept of improving cards is more widely accepted in the hobby than it was even a few years ago.
If TPG is not seeing evidence of the improvements, the question is, should they? And if not then is the work just that good or is that work too subtle to be detected. Evidence should not just be applied to sloppy work but if a tree falls in a forest...