In one perspective that is right. But what I understand is that EBG suppress the propagation and confine it to region in between to form SIW? Refer to the paper: Hill, Michael J., Richard W. Ziolkowski, and John Papapolymerou. "A high-Q reconfigurable planar EBG cavity resonator." Microwave and Wireless Components Letters, IEEE 11.6 (2001): 255-257. This paper describes the configuration as EBG cavity, and for me it looks like SIW too, so why two different terms?