I don't know the legal criteria for public disclosure, if any. I would think the offer would have to be permitted under Ariad's corporate governance, likely have an effect on the share price if disclosed, etc.
Fact, we know HB has tragically over-played his hand in the past with the FDA. IMO, could be the case with suitors, too. That reduces appeal. Ariad has real strengths and real problems, so the buyer would be purchasing both. This reduces to the pool of buyers, eliminating those who really don't want risks or fund solutions and deal with the issues.
IMO, not on suitors list: PFE, AZ, NVS, MRK