;; Use the Emacs standard indentation binding. This may upset c-mode
    ;; which does not follow this at the moment, but I see no better
    ;; choice.
    (define-key map [tab] 'indent-for-tab-command)

That last line should be:

    (define-key map (kbd "TAB") 'indent-for-tab-command)

I can see that the original code correctly used TAB (\t) instead of <tab> ([tab]) and it was later changed (commit 3699aca); but I strongly suspect that change was made for purely aesthetic purposes, as it's a bug.

Terminals don't send <tab> events; they only deal with TAB. This is why Emacs (a) translates <tab> to TAB, and then (b) uses TAB consistently for binding keys (and certainly the referenced c-mode-map binds TAB and not <tab>). This convention ensures that GUI and terminal Emacs always do the same thing.

With the current code, C-h k TAB tells us:

• <tab> runs the command indent-for-tab-command in GUI frames
• TAB runs the command c-indent-line-or-region in terminal frames