Well, Mike has also admitted that "it can be argued pretty strongly that Wikipedia is not currently complying with GFDL fully", in which case it's irrelevant, because the GFDL is automatically revoked upon such non-compliance anyway.
That may be an issue, but it's a far less simple one than openly using content under a license it hasn't been released under.
In the end, all that really matters is that the people who own the copyright to Wikipedia aren't interested in going through the hassle and expense of suing the WMF.
Wikipedia has made plenty of enemies over the years, some of them fairly wealthy. I think there is a good chance of someone deciding to cause trouble if they think they can win. It only takes one.
And if any of them actually were, the WMF would probably be willing to remove their contributions way before it got to that point anyway.
That's easier said than done. It would probably end up requiring the deletion of any article they contributed to, unless someone's willing to take the time to go through each article and just delete the parts they specifically contributed to.