The following passage is quoted from the article in Newsweek by Leah McGrath Goodman, titled "Thomas Piketty Says He Was Ambushed":
[Financial Times’s economics editor Chris] Giles, in his critque, takes issue with everything from what he suggests are innocent errors in the transcription of Piketty’s data to numbers that “appear simply to be constructed out of thin air.”
Most damaging in the assessment is his assertion that Piketty’s data on rising wealth inequality may be incorrect, holding that this contention is “the centerpiece of Capital in the Twenty-First Century.”
What is the subject of the verb "holding"? I think Giles is.