"He {Scalia} pioneered 'originalism,' a theory holding that the Constitution should be interpreted in line with the beliefs of the white men, many of them slave owners, who ratified it in the late eighteenth century."

Truth is, I'm not crazy about his use of the slaveholding factoid there either, but it clearly isn't implying that "therefore their ideas can be ignored," as you suggested. Rather, I take it to suggest that the founding fathers' understandings (especially on social issues) are not always prescriptive. It's a critique of Scalia, not Jefferson.