Do you have sources on this? The conventional wisdom in the community is that it's the SC2 engine -- a view backed up by the game's origins as an SC2 map, similarities in internal file formats, folder structure, the nature and availability of video settings, and this 2014 article by Polygon that confirms it outright [1]. Other articles from 2014 [2][3] talk about how the same team handled the development-until-release of SC2 and Heroes of the Storm.

[1] http://www.polygon.com/2014/5/21/5735230/heroes-of-the-storm... [2] https://www.icy-veins.com/forums/topic/6778-heroes-of-the-st... [3] http://heroes.blizzplanet.com/blog/comments/heroes-of-the-st...

It does use the same engine as StarCraft II. It originally started as Blizzard All-Stars and was supposed to be a custom map for the SC2 arcade, but the team ran into technical roadblocks that pushed them into making HotS a separate game.