I doubt Missling got any offer from BIIB or that he would have turned down a reasonable offer. If the study had been successful and BIIB had made an offer that Missling would not take, Biogen would have bought the company at a substantial premium to the stock price at the time. Also, I can’t imagine Anavex would have entered into the MTA without at least having an agreement in principle with Biogen on the terms of a collaboration if the remyelination study proved successful. I think the most likely result was that Biogen felt the study results did not warrant further investigation and the MTA ended.
As a former long term investor in BIIB (ended ~10 years ago) I can recall articles outlining how BIIB at that time had established a reputation for low balling offers for partnerships or other M&A activities. These articles weren't saved so this should go down as IMO.