I don't buy this theory. If it were true I think we'd have strong evidence that CoinLab actually received $75 million. And if those coins were sold in May 2013, I expect we'd have seen a much sharper decline in the bitcoin price. EDIT: and most importantly, if this were the case, Mark would have undeniably been illegally operating an insolvent business--even CoinLab would have known this. Word would have gotten out sooner.
Apparently the lawsuit is still in progress and is scheduled to be tried in Nov/2014, may drag on to 2018:
https://bitcointalksearch.org/topic/m.5261573
Reply by MtGOX to the lawsuit:
http://numismatics.pwnz.org/coinlab-v-mtgox-answer-complaint/
Reply by CoinLab to MtGOX's reply:
http://thegenesisblock.com/coinlab-files-response-mt-gox-lawsuit/
Lawsuits are often settled out of court, but if that had happened I suppose that we would have known.
However, if MtGOX had settled the lawsuit by handing over 750,000 BTC instead of 75 M$, CoinLab obviously would not denounce him.