Linux distros are free to choose which standard libraries and toolsets they use. Android has chosen not to use the standard GNU coreutils or glibc, they instead use toybox and bionic.

There are other distros that use for example musl and busybox over glibc and coreutils. Ubuntu is also looking to replace coreutils with a new rust version that isn’t copyleft. There are also multiple init systems out there so you can’t say that systemd must be used to consider something a Linux distro. Etc.

I’m wondering, what exactly about Android doesn’t count as a Linux distro to you?

That’s literally what makes it a Linux distro… It uses the Linux kernel.