30 Amp Cooktop on 40 Amp Breaker
The term they are supposed to use is Minimum Circuit Amapcity (MCA) and/or Maximum Over Current Protection (MOCP). In your case the MCA is 30A, but they apparently didn't provide an MOCP, just a "suggested" one. I think you are fine with a 40A breaker, so long as your conductors in the wall to the receptacle are rated for that and at #8, they are.

The rating on any circuit in the NEC is based on "continuous" current, defined as 3 hours or more. On a stovetop, it's highly unlikely that you would have ALL burners set at FULL output for 3 hours or more.