I think OTP Releases would still depend on the target host being configured correctly and having all dependencies installed that your application needs to run.

Containers are all about consistency of the environment. For example, maybe your Elixir app wants to log out to something like Splunk or DataDog...if those agents aren't installed on the host, it doesn't matter what facilities BEAM provides. Or maybe your app depends on certain configs, libraries, or files being available at specific paths on the system. Things like that would be captured in the image your containers are running and guarantee consistency across machines and environments.

My bad, I sould have specified, I was only addressing "Without Docker, you might have a different version of BEAM installed (or none at all)"

Ah, makes sense!

Right, they do, that is why containers are still useful to releases for deployment.

you still need to compile your system on the same combination of OS/libc

libc builds don't work on Alpine

Correct, but you still need to compile the release on a similar system.

For example if your dev machine is Debian it won't work if the deploy machine is Alpine (similarly Windows builds won't work on Linux or Mac).