That said, newer Androids use seccomp to restrict which syscalls you can use, basically to what bionic exposes anyway. This doesn't seem to affect Termux and friends, which can apparently run full X11 applications without root.
(edit) Notably, splice() is still callable, so maybe the POC needs to be tweaked...
As for termux,
Someone can statically build a freestanding executable/so targetting arm64 linux (specifically the right android linux kernel version) and it will run fine on Android. The syscall interface, process model, file descriptors, signals, memory mapping, all of this is Linux, this is what people mean when they say Android is just Linux.
Also some obligatory Linux vs GNU/Linux comment. (and it's not like GNU/Linux doesn't ever change under your feet - see the glibc DT_HASH debacle)
Google relies on Linux LTS kernels. When the Linux LTS team dropped support from 6 years down to 2 years, Google stepped in to cover the 4-year gap.
It is Linux. It's basically a distro.
- Is totally Linux