I have no direct experience with fluidized bed gasification, I am only aware of it conceptually. From the little I have seen or read I would assume they mean the bed can be fluidized with that percentage of air and achieve gasification. The straw is going to be quite light relative to the bed, so it seems plausible that fuel is metered in to manage gasification at an appropriate rate, the fluidization is in a way a separate design issue.
There must be some technical issues with it, if it was simple there should be more systems. Then again, with energy and fuel as cheap and accessible as it has been, maybe the incentive wasn’t there.
A swirl burner could be the simplest combustion approach, very little technical challenge once the appropriate design is established.
As you say, the field of efficient straw combustion seems to be pretty unexamined. It sounds like a fun area for experimentation, the fuel is over abundant, extremely cheap and easy to get, dry and ready to burn, processing minimal.