It does use Widevine. Sling is basically the Move Networks technology EchoStar (DISH) bought three or four years ago, combined with some amendments for digital to EchoStars existing distribution deals.
Heck, if you check DNS they are even still using the old Move CDN configs:
~> host www.sling.com
www.sling.com is an alias for cdn.prod.movetv.com.c.footprint.net.
Heck, if you check DNS they are even still using the old Move CDN configs: