Although if this happened and legitimate code was rejected by IBM, what could happen is a community backlash against IBM as a maintainer and the code could enter that way.

I'd assume the person has received confidential information about the IBM technology in question during his employment. Of course the Linux driver is open source so it cannot be a business secret. But typically there is much more knowledge than just the Linux source code. Using business secrets after your employment is most likely enforcable in many legislations. Damages or criminal charges or both.

Such an action would only decrease developer goodwill towards IBM. Needless to say, IBM is already not one of the "cool" companies to work for.

there is this stipulation of using a clean room in which to introduce your idea in order not to be sued for “stealing” intellectual property.

And often times, the same person cannot be used in that clean room.