Graft's definition sounds more like a Git "commit" than one found in a SQL standard. Perhaps that's the source of this confusion?

That may be the case, but the Git notion of a "commit" is a domain concept that is unrelated to, and incompatible with, the distsys notion of a "commit". And terminology like "strict serialization" and "snapshot isolation" and so on -- which this system and its docs reference -- are distsys concepts, not Git concepts. So "commit" needs to abide distsys rules, not Git rules, as you can't really mix-and-match definitions across domains.

Git is a DVCS. The D stands for distributed, meaning (in old people language) masterless. Git doesn't have a "global."