Two problems:

- `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. that of the original Org mode file. However, you need to turn off `org-indent-mode` not in the original buffer but in the generated indirect buffer. Fortunately, it saves that indirect buffer in the global variable `org-last-indirect-buffer`, so you can use `with-current-buffer` to temporarily switch to the indirect buffer and disable `org-indent-mode` there.

The final form of the function would then look like this (EDIT: the fontification of the buffer was lost when executing the original form of the following function, so I added an explicit
call to `font-lock-fontify-buffer` to restore it, although this is probably a workaround, not the real solution. However, I *still* cannot reproduce the OP's problem where he loses `org-indent-mode` in the original buffer) /EDIT):

```lang-le
(defun my-org-to-indirect-buffer ()
  "Run `some-command' and `some-other-command' in sequence."
  (interactive)
  (org-tree-to-indirect-buffer)
  (with-current-buffer org-last-indirect-buffer (org-indent-mode -1))
  ;; the original buffer loses fontification for some reason, so we restore it explicitly
  (font-lock-fontify-buffer))
```

One more problem might be the key binding: why do you bind it in the global map? Presumably you only need it in an Org mode buffer, so why not restrict it to the mode-specific keymap? Something like this:

```lang-el
(define-key org-mode-map (kbd "C-c f") #'my-org-to-indirect-buffer)
```