Their belief system is absolutely in line with Jewish cosmology c.600 BCE. One has to selectively quote to get those ideas to fit into a heliocentric solar system, so forming an explanation for the potential of other universes is really well beyond the pay grade of WBTS theology.
The focus of Judaism back then was on their God doing things on Earth. Busy God smiting foes, rewarding worshippers, listening to prayers, interacting with other Gods etc etc. His whole attention is centred on Earth. Even when not doing anything, he's chilling out in heaven with his angels, waiting for Satan to pop by and make a bet about an Earthling. It's God as Heavenly King, doing most of the things which Earthly Kings do. The Jewish God isn't so different to any others from that time. One Jewish group even have him popping down to earth to get a woman pregnant. Just the one is quite restrained by God standards to be fair. Zeus couldn't keep his pants on. But there's no conception of there being anything other than Earth and an invisible place God (and other spirity critters) lives in. The JWs have taken that idea and ran with it, which creates the obvious problems 2600 years later.