A spay can be done either by removing the ovaries alone or the ovaries and the uterus. If a cat has no ovaries, she's spayed - end of story.
Pyometra is caused by repeated heat cycles that leave the uterus permanently somewhat engorged. This tissue is fertile ground for infection. A cat without ovaries cannot experience heat cycles and therefore is not susceptible to pyometra. Incidentally, pyometra is still quite rare even in cats who have had many heat cycles. It is much more common in dogs.
Theoretically, any body part can become infected, but under these circumstances removing your cat's uterus would be sort of like removing your own healthy appendix because it might possibly get infected someday (except that you are probably more likely to get appendicitis than your cat is to get pyometra!). Surgery itself is not risk free and IMO the benefit in no way outweighs the risk in this case. My female cats were spayed by oophorectomy (removal of the ovaries alone) rather than ovariohysterectomy (removal of uterus and ovaries), as were many of my friends' cats, and after many years none of us has had a single problem associated with it.