I guess it varies from person to person because I consider Android and webOS to be Linux. It's really just strong branding.
The definition of that does seem to be a bit hazy in this era. Calling something a linux distro implies to me that it's a more pure form of the OS without as much customization or proprietary layers on top. And it also feels more like something that runs on a traditional PC as a desktop OS as opposed to a heavily modified version for a specific class of hardware other than PC's. While webOS and Android both use linux at the core, the user doesn't really see it or have a need to dive into it. All the apps and user-facing features are built in a layer of web code or java, respectively.
I usually think of the term "Linux distro" as meaning GNU/Linux. "Linux-based" seems to be the preferred term for just anything that uses the Linux kernel.