> I think you may misunderstand: setq always sets a global variable.

Since when?

  -- Macro: setq-local variable value
     This macro creates a buffer-local binding in the current buffer for
     VARIABLE, and gives it the buffer-local value VALUE.  It is
     equivalent to calling `make-local-variable' followed by `setq'.
     VARIABLE should be an unquoted symbol.

Note the last sentence, which points out that `setq' does not always
set a global variable.  If it did, then so would `setq-local',
according to that text.