BP don’t calculate future valuation in their “heart of hearts” nor need they do it based upon market cap premium. They do it based upon future projected sales.
But you said:
Merck cannot simply pay $20B for NWBO, whose current market valuation is under $700M, because they just know in their heart of hearts that NWBO is really worth it. They have to be able to justify that price to their investors. Someone in the C suite would likely get fired if they were bring an offer like that to investors. It's not happening.
I'm not disagreeing with you that NWBO may be worth that much. I'm not disagreeing with you that BP may know that.