The link has a stray forward slash ("/") at the end of the link, but otherwise has no problems. Slide 20 of the correct resource says (in graph form) that "81% of covered workers in firms with 200 or more workers are in partially or completely self-funded plans", which is indeed not what the text on Wikipedia claims. A footnote on the same slide says "Sixty-one percent of covered workers are in a partially or completely self-funded plan in 2014." (Please accept my apologies for not fixing these two issues on Wikipedia.)

The 2017 edition of the underlying report is available at https://www.kff.org/health-costs/report/2017-employer-health... . The report states multiple times that "Sixty percent of covered workers are in a self-funded health plan." The following portion is relevant to your discussion a few comments up: "Eighty-one percent of covered workers in firms with 1,000 to 4,999 workers and 91% of covered workers in firms with 5,000 or more workers are in self-funded plans in 2017". Figures 10.1 and 10.2 show these numbers in graphical form.