I suppose it had to be clarified. You wouldn't expect to see both rendering engines there, would you? Unless, of course, you're going to get two browsers from Microsoft, catering for the legacy aspects, and the other for the future of browsing.
I'm still slightly confused, but no surprise there, eh! ;)

Back in January 2015, Microsoft's plan of record was to include both its existing "Trident" (MSHTML) and its new "Edge" rendering engines in both IE 11 and Spartan. That way, whether customers were using IE 11 or Spartan, they'd be able to switch back to the legacy Trident engine if and when they encountered legacy or enterprise sites that needed Trident to render correctly.

But based on feedback from Windows Insiders and customers -- who've been able to test the new Edge engine even though Spartan itself isn't yet available in publicly available test builds of Windows 10 or Windows 10 Mobile -- Microsoft is shifting gears.

Microsoft's new strategy for Windows 10 browsing is for Spartan to include the Edge rendering engine only and IE 11 to include the Trident engine only. Microsoft Clarifies Rendering Engine Changes For Spartan Browser [zdnet.com]