If I open a random file (in this case, say .zshrc) in a blank vim with no rc file , but with vi compatibility forcing off

vim -N -u NORC ~/.zshrc

As expected vim will launch and display .zshrc without syntax highlighting.

attempt to enable syntax highlighting

I have a file containing a collection of highlight commands called darcula.vim in

~/.vim/colors/darcula.vim

contents of ~/.vim/colors/darcula.vim

"
" Darcula colorscheme for VIM.
"

set background=dark
highlight clear

if exists("syntax_on")
  syntax reset
endif

let g:colors_name = "darcula"

hi Cursor ctermfg=188 ctermbg=250 cterm=NONE guifg=#2b2b2b guibg=#a9b7c6 gui=NONE
hi Visual ctermfg=NONE ctermbg=24 cterm=NONE guifg=NONE guibg=#214283 gui=NONE
hi CursorLine ctermfg=NONE ctermbg=236 cterm=NONE guifg=NONE guibg=#323232 gui=NONE
hi CursorColumn ctermfg=NONE ctermbg=236 cterm=NONE guifg=NONE guibg=#313335 gui=NONE
 
[ ... plus many more highlight commands ...]

at vims command line I enter

:colorscheme darcula

then, to check if the highlight commands have been loaded, I run

:hi

and yes vim is showing all the hi commands from the above darcula.vim file have been loaded.
Then I run

:syntax enable

and nothing happens

from reading :h syntax-loading I thought the syntax enable command sets up filetype autocmd recognition, as verified with:

:filetype
filetype detection:ON  plugin:OFF  indent:OFF 

and also sets up autocommand which will run the vim function SynSet() on the autocommand event Syntax, from the file $VIMRUNTIME/syntax/synload.vim

[...]
au Syntax *     call s:SynSet()

fun! s:SynSet()
  " clear syntax for :set syntax=OFF  and any syntax name that doesn't exist
  syn clear
  if exists("b:current_syntax")
    unlet b:current_syntax
  endif

  let s = expand("<amatch>")
  if s == "ON"
    " :set syntax=ON
    if &filetype == ""
      echohl ErrorMsg
      echo "filetype unknown"
      echohl None
    endif
    let s = &filetype
  elseif s == "OFF"
    let s = ""
  endif

  if s != ""
    " Load the syntax file(s).  When there are several, separated by dots,
    " load each in sequence.
    for name in split(s, '\.')
      exe "runtime! syntax/" . name . ".vim syntax/" . name . "/*.vim"
    endfor
  endif
endfun

[...]

which looks like it will use the filetype and run the appropriate syntax file.
To me this looks like Ive covered everything, but I obviously havent, why isn't syntax highlighting happening here?