Like most urban legends, this one might have some basis in fact.
Way back in the day, some ATF formualtions contained whale oil, an ester.
Since esters are highly polar and the motor oils of the day had little detergency, these ATFs might have had a cleaning effect.
I wouldn't use ATF with any modern oil and anticipate that it'll do anything positive.
Having said that, you can find a few posts on the site in which members swear that they've used ATF as a cleaner with good results.