An OS is the interface layer between hardware and software. It’s the first code that runs after the boot loader, and it exposes an API for syscalls that allow user processes to allocate typically restricted resources, while also tracking and maintaining those allocated resources, doing process scheduling, and a bunch of other critical tasks.

All distros are operating systems because they ship all the tools and utilities need for the system to function

All distros contain operating systems (or, more accurately, kernels), or, rather, are built on top of them. A distribution is a collection of curated software, along with an init system and, for linux, package manager, and, frequently, a particular desktop environment. These pieces of software are, on some level, superfluous. You can have an OS without them. They don’t comprise the OS as a distinct conceptual layer of a computer system, of which there is the hardware, operating system, application, and user layers. The operating system is just Linux - because that is the interface layer between the hardware and software.

Saying “all distros are operating systems” is like saying “all cars are engines.” It’s just wrong. And I don’t care what wikipedia has to say about it.