Big thanks to NickD for setting me on the right path.
Indeed, the error message "Not enabling jit-lock: it does not work in indirect buffer" is key to this problem.
When looking up that error message in jit-lock.el, we find this comment:
;; We're in an indirect buffer, and we're turning the mode on.
;; This doesn't work because jit-lock ...
org-indent-mode is a minor mode and like all minor modes is enabled by calling with a positive argument and disabled by calling it with a negative argument - see its doc string with C-h f org-indent-mode. So you have to call it like this: (org-indent-mode -1) to disable it.
org-to-tree-indirect-buffer leaves the current buffer unchanged, i.e....