What is a machine ID in this case? Surely the useragent isn't exposing the MAC address of the user. Is it just a first party cookie added to the current browsing session that is meant to be a facsimile of a "machine ID"?
I presume, to start with, it's the ID of the server generating the snowflake, not the client.
After that I guess it's up to the owner of the servers (and the particular snowflake implementation) how they assign machine IDs.