Because it’s essentially setSeed(getTimeMillis()). V6 and v7 are sortable, that’s why they exist. Which means like getTimeMillis() there are a finite number of starting points to try to guess the seed.
v6 is a transform of v1 UUIDs to behave like v7 keys with respect to database indexing - increasing over time. If it’s a function of time, then it’s still guessable by brute force.
Another responder suggested that the mention of v6 UUIDs is an error. Maybe. But that’s a truly bizarre typo to make. And they still haven’t fixed it.
