From the Time article: 'The [New York] Times found that nearly 20% of Trump supporters did not approve of freeing the slaves, according to a January YouGov/Economist poll that asked respondents if they supported or disapproved of “the executive order that freed all slaves in the states that were in rebellion against the federal government”—Abraham Lincoln’s Emancipation Proclamation.'
Of course, the more detailed results from snopes.com says, "Of the 2,000 respondents, 53 percent said they strongly approved; 17 percent approved somewhat; 8 percent disapproved somewhat; 5 percent disapproved strongly; and 17 percent said they were not sure." So, 13% percentage disapproving was rounded up to "nearly 20%", but curiously the more accurate assertion that 30% did not approve was not used.
Furthermore, the snopes.com suggests from related polling that perhaps the opposition was more towards executive orders in general, i.e., opposition to the means of freeing the slaves rather than the actual freeing of the slaves. It also suggests that liberal Republicans were more likely to be white supremacists than conservative Republicans and that similar survey results were seen before Trump's presidency. Also, without a comparable survey of Clinton supporters, it's not clear that the results are not simply reflective of Americans in general.