While they are related, and the terms are somewhat overloaded and depend on context, I wouldn't call SDF ray marching (aka SDF sphere tracing) a subset of ray tracing, at least not according to today's common usage of those terms. Unless you are using 'ray tracing' to mean the family of techniques that do anything with a ray (which is not the most common interpretation, IMO), then there are things each technique can do that the other cannot. Ray tracing most commonly refers to solving ray-surface intersections directly, and/or checking visibility strictly between two points in space, one or both of which may be infinitely far away. SDF ray marching to a surface is iterative and doesn't normally solve ray-surface equations directly, nor does it generally give you a surface normal. Ray tracing, in contrast doesn't allow for neat tricks like single-sample soft shadows, because it doesn't give you any other information about the scene except what's along the ray.
While I wouldn't insist on it, I think you could even argue that it's the other way around, ray tracing could be seen as a subset of ray marching, because it's possible to build a ray tracing query out of ray marching, but not possible to build a ray marching query using ray tracing, for example, the kind of basic ray marching you find on ShaderToy can tell you by how much you almost hit a surface, but vanilla ray tracing can't.