Linux is often used to refer to a family of operating systems including Ubuntu, Debian, fedora, red hat, ect., which all use the Linux kernel.
However, GNU/Linux may be a better name for this family of operating systems, since they all use GNU components and (to varying extents) embrace the philosophy of the free software foundation.
Android uses the Linux kernel, but not GNU components, and do not embrace the philosophy of the Free software foundation.
Stalman, the man who founded GNU and the free software foundation published his thoughts on this:
https://www.gnu.org/philosophy/android-and-users-freedom.en.html
Yeah, but it’s not like a typical Linux installation on a PC would be. You can’t just install a Flatpak application or anything like that. It doesn’t use many of the GNU core utilities that most other Linux distros use, and doesn’t use a mainline kernel.
People that ask for Linux phones know Android is Linux.
It’s just a lot more concise to say than “I want a phone with an open bootloader and hardware fully compatible with a mainline Linux kernel. I want to have a phone that can run a Phosh/Gnome Mobile/Plasma Mobile UI and on the backend work in a similar way to how desktop Linux would.”