Being able to practially develop anything without running Google's proprietary build of Android SDK can be enough for devs to see the difference.
It might be very similar on the surface, for casual person installing app from appstore. But the whole ecosystem above the Linux kernel and below app interface is barely comparable. No Flatpak, Wayland, SystemD, glibc, PipeWire, etc.
The way apps are build, most of the times does not matter what kernel is at the bottom. What matters are toolings and the ecosystem.
Android could be build on Windows NT and most won't notice (remember Windows Phone? Not looking at UI/UX design, it felt really similar), yet we wouldn't call Android as being the same as Windows.