I suspect Idle just passes control characters directly to an underlying Text or similar TK widget. As far as I know, TK only documents behaviour for tabs and newlines, not other control characters.

Last time this was brought up, Terry added a sentence under <https://docs.python.org/3.7/library/idle.html#user-output-in-shell>, third paragraph, about this:

“Newline characters cause following text to appear on a new line, but other control characters are either replaced with a box or deleted.”