Strange that it's not on the website there. I don't know if the investment really happened either, but there are a couple links on a16z which seem to suggest they did invest in TopTal. Some articles reference a seed round.
(these links are all empty apart from a logo and a date ranging from 2013 to 2016)
(these links are all empty apart from a logo and a date ranging from 2013 to 2016)
https://a16z.com/toptal-2/
https://a16z.com/toptal-logo-3/
https://a16z.com/toptal-logo/
https://a16z.com/toptal/