Each SSA office deals with different types of noncitizens, and some may prove their eligibility with their I-94 (for example, temporary visitors, temporary workers who entered with an employment visa), but others may prove their eligibility with an EAD, or some other document. You are not required to have your I-94, the worker who asked about it may have just been reacting to his own past experiences with people who used I-94s to prove eligibility. Good luck!
The information above is general in nature and is not intended to create an attorney-client relationship between us. It is intended simply as background material, is current only as of its indicated date, and may not include important details and special rules that could be applicable to your case. You should consult an attorney directly before acting or refraining from action.