They need not have been from the wikileaks source.
One can try to erase “sent” copies of 33,000 emails, but for each one sent there is one “received”. Trying to erase/destroy all of the emails that were received would be a monumental chore and even if you could track down each and every one of them, you cannot be 100% certain the recipient didn’t make a cya hard copy prior to erasing as per instructions from superiors.
My guess is that some of these “received” emails were located (or anonymously sent to the FBI), the content of which contained classified information or otherwise pertinent to the the original case.
That’s exactly what Mike Cernovich says. He says it got this from an FBI source:
From his twitter Nov 12 (@cernovich):
I can confirm rotten Hillary’s 33,000 emails have been obtained. Word is it will be a “November surprise.” Wow, something new every day!
From his twitter today:
Told you somebody big was coming. FBI has all her emails. Don’t ask me how I knew that.