It means that if someone breaks out of your container, they can only do things that user can do.

Can that user access your private documents (are these documents in a container that also runs under that user)?

Can that user sudo?

Can that user access SSH keys and jump to other computers?

Generally speaking, the answer to all of these should be “no”, meaning that each group of containers (or risk levels etc) get their own account.

The linked article — and others — explain that in Android 10+, (a) executable binaries can no longer reside in a read/write directory, and (b) access to /sdcard will go away. Simply put, these changes destroy my application’s ability to function, and that of Termux as well.

That sounds like proper security to me? Inability to access the user’s storage is a bit lame, but they’ve been moving to nicer APIs for that anyway.

Android is a mobile phone OS, not desktop / embedded Linux.