In the org manual the following lines can be read about #+SETUPFILE:
(emphaze is mine):

» Org also parses and loads the document during normal exporting process.
» Org parses the contents of this document **as if it was included** in the
» buffer.  It can be **another Org file**.

However, when I consider the two following files, code block in
setup.org is not evaluated at all when exporting, whereas when
#+SETUPFILE: is replaced by #+INCLUDE: it is.

--- main.org  ----------------------------------------------------------
#+setupfile: setup.org

Something
------------------------------------------------------------------------

--- setup.org  ---------------------------------------------------------
#+begin_src emacs-lisp :exports results :result silent
(message "DEBUG")
#+end_src
------------------------------------------------------------------------


Emacs  : GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-03-28, modified by Debian
Package: Org mode version 9.4 (9.4-elpa @ /home/bruno/.emacs.d/elpa/org-9.4/)

-- 
Bruno BEAUFILS
Trésorier de la Societé Informatique de France