As an European: cloud is someone else computer, sovereign computing means users own their iron, sw and data, as government, so a sovereign computing means a State own hw, sw and data belonging to it.
That's ALMOST the case for most EU states so far, but less and less the case, and more and more with private partnership engendering public IT, which is public information, nervous system, witch is the OPPOSITE of sovereign computing and Gaia-X (a failed project anyway) it's the apex of such disgraced model. Oh BTW to be sovereign ALSO DESKTOP must be FLOSS, witch is almost not the case in any public administration. The hw since it's full of fw to the point of being de facto connected black box, network hw included, mush be open or state-made. Witch is not the case in the 99.9% periodic of the cases.