[Q] Why does sourcing fzf-tab.zsh from a script cause syntax errors?

Describe your question I maintain a Bash script which sets up my shell configurations, useful when working on a new server for the first time.

I recently added fzf-tab to this script, which I execute like this:

One of the things the script attempts to install is fzf-tab, as per the following excerpt:

ZSH_CUSTOM="/<redacted>/oh-my-zsh/custom"
FZF_TAB="${ZSH_CUSTOM}/plugins/fzf-tab"
if [[ ! -d "${FZF_TAB}" ]]; then
  git clone https://github.com/Aloxaf/fzf-tab "${FZF_TAB}"
  source "${FZF_TAB}/fzf-tab.zsh" && build-fzf-tab-module
fi

However, when the script gets to that block, it shows errors stemming from your module:

Cloning into '/<redacted>/oh-my-zsh/custom/plugins/fzf-tab'...
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 2: local: can only be used in a function
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 6: builtin: setopt: not a shell builtin
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 24: syntax error near unexpected token `)'
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 24: `    || ({ -ftb-zstyle -m disabled-on "files" } && [[ -n $isfile ]]); then'

Passing the -x flag into the script and rerunning generates the following slightly more informative output:

Cloning into '/<redacted>/oh-my-zsh/custom/plugins/fzf-tab'...
+ source /<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh
++ builtin local -a _ftb_opts
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 2: local: can only be used in a function
++ [[ ! -o aliases ]]
++ [[ ! -o sh_glob ]]
++ [[ ! -o no_brace_expand ]]
++ builtin setopt no_aliases no_sh_glob brace_expand
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 6: builtin: setopt: not a shell builtin
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 24: syntax error near unexpected token `)'
/<redacted>/oh-my-zsh/custom/plugins/fzf-tab/fzf-tab.zsh: line 24: `    || ({ -ftb-zstyle -m disabled-on "files" } && [[ -n $isfile ]]); then'

Strangely and importantly, when I source that file interactively - so manually, not inside a script - it seems to run just fine.

Any idea what the issue might be?

For context, all code and outputs above were obtained from the master branch, commit c2b4aa5ad2532cca91f23908ac7f00efb7ff09c9.

Aloxaf / fzf-tab

[Q] Why does sourcing fzf-tab.zsh from a script cause syntax errors? #406