Well, no. They would be inadvertently source-available. If the license doesn't comply with the Open Source Definition, it's not open source.
EDIT: Downvoted as usual for correcting false impressions about how free software and open source works.
Are you seriously suggesting that the OP you responded to was comparing "stolen" or "leaked" software to open source? That is probably why you're being downvoted, not because people around here have a false impression as to what constitutes open-source.
Moreover, it reflects a critical flaw in the open source dogma as compared to free software. Open source puts source code at the forefront, which is fallacious. The key elements are the ability to run unfettered, study, modify and redistribute identical or modified versions. The source code is a necessary precondition for properly exercising those freedoms, but not a central focus in any real sense. It is easy to misinterpret "open source" as being about publicly viewable source code, and many companies have exploited this to their advantage presently or in the past (GitHub with Atom, Epic Games with UE4, etc.)
The OP was explicitly calling such stolen or leaked software open source. Please read more closely.
It would be more likely to just have handful of high visibility repos.
That being said, I'm also curious about GitHub's efforts to prevent such a scenario.
"We use better than industry standard encryption on private repo data, encrypted with AES-1024 and encrypted in transit via TLS-3 etc etc..."
But, I don't know. What was that quote? "Don't put anything on the internet that you wouldn't want to see on your grandma's coffee table."
Do we have any info on what steps github takes to prevent this? I ask as a paying customer (with both personal and corporate accounts).