Board Thread:Episode Discussions/@comment-1181222-20180131111324/@comment-33351429-20180131191622

Looking at "Release" pages, it mentions that several of Release users have different forms of of that release. I.e. Mei uses Acidic Mud, while Kurotsuchi uses ashes and quicklime for Lava Release. So it's very possible that different variations of the Release is what makes it a kekkei genkai.