It will see a string delimited with ':'. The point is that while inside xonsh, your variable is typed, and then xonsh hands the env in the expected way to the subprocesses.
So basically it goes back to string from array by simply unconditionally joining it on a ':'? I guess that makes some sense... But isn't this convention (delimited by ':') only for PATH?
The convention holds for other lists too (usually lists of directories). The important thing is that there is a similar but different convention on Windows (';' is used), and xonsh abstracts that away in the same way Python does.
