I think she understood the consequences but thought that the risk was worth the reward. In some cases I think personal hubris was involved.
I don't do Billion dollar deals but I've done my share of 7 figure ones. At some point in every negotiation you need to tell your attorney that you understand the risks and you're willing to accept those risks or nothing would ever get signed. I think Marissa was convinced the Mozilla deal would be huge for Yahoo. She likely didn't think or care if the company got sold (honestly in that case she's probably out so there's no personal risk for her) and she was convinced DeCastro would be able to do at Yahoo what he was able to do at Google.