You have contradicting information in your problem statement. You first indicate that the client offered very high level requirements with the understanding that your firm would learn his business and solicit additional requirements. Then you indicate your bosses are asking you to skip that work and proceed to develop the product. It sounds like your client wants to collaboratively breakdown the requirements but your bosses don't. So your bosses are difficult, not your client, right?
If the effort to build this product without soliciting other requirements is being paid for by the client, then producing the product without working with the client to decompose the specs would be a huge mistake, a huge risk for your firm. As others have said, raise that risk. It doesn't mean they have to or will listen, but you need to put it on record. Then, do as they say.
If the effort is not paid by the client and the firm just wants to put something together to start conversations and maybe show off your firm's capabilities, then proceed. Perhaps interview some of your colleagues who may know some more detailed information about your client will help you breakdown those specs yourself with some degree of accuracy. After all, you indicate the client is well known to your firm. Since the client did not pay for it, then it becomes a starting point for the future requirements sessions.