これは、マニュアルにかなり明確に文書化されています。
INTERACTIVE (-i, ksh: -i)
This is an interactive shell. This option is set upon
initialisation if the standard input is a tty and commands are
being read from standard input. (See the discussion of
SHIN_STDIN.) This heuristic may be overridden by specifying a
state for this option on the command line. The value of this
option can only be changed via flags supplied at invocation of the
shell. It cannot be changed once zsh is running.
ただし、エイリアスの展開を防ぐためだけにインタラクティブ モードをオフにする必要はありません。
$ setopt no_aliases
$ . ./foo.zsh
./foo.zsh:3: command not found: a
$ setopt aliases
$ . ./foo.zsh
ha