They're saying that for a sequential number, you must have a central authority managing that number. If two environments have different authorities, they have a different mapping of these sequential numbers, so now they can't share data.
Oh. Like, if you have two different schema registries that you simultaneously deploy new schemas to while also wanting to synchronize the schemas across them.
There is no difference between a random 128bit integer vs a sequentual 64bit integer except that the 64bit integer is smaller.