Wikileaks claim that they were leaked.
Believe who you want. There's probably nothing that will convince either of us otherwise.
Those who say they were hacked and who have provided evidence of such: FBI, CIA, NSA Crowdstrike, Mandiant, Secureworks, Mueller Report, Republican Senate Report, Democratic Senate Report, Dutch Security Services, UK Security Services, the Russians themselves, Donald Trump himself.
Your inability to be persuaded isn’t an asset here. Seriously, read the Muller report and Crowdstrikes attribution. It’s persuasive beyond a reasonable doubt if you’re moderately technical.
It’s actually what SecureWorks and other private companies said, SecureWorks is a private company that is a subsidiary of Dell but I’m sure you’ll find some other reason not to believe it.