it's in the format /grist get, but /grist is in the format /grist get

The grist command originally supported that uuid format for players that were not online (hence why it was used in the data checker)
When minecraft refactored its command system into brigadier, I lazily ported the command to use the minecraft-provided player argument, rather than create an argument type for our player identifiers

Oh, and yeah there's the changed argument order that you pointed out