Comment by skissane

IANAL, but they likely believe their unpublished source code contains trade secrets. They may believe that training a public model is okay on published source code (irrespective of its copyright license), but that doing so on unpublished source code containing trade secrets might legally count as a voluntary relinquishment of their trade secrets (if we are talking about their own code) or illegal misappropriation of the trade secrets of others (if they trained it on third party private repos)