I don't know. You'd need quite a number of threads + machines generating uuids in the exact same microsecond to get an opportunity for a collision. It doesn't seem obviously false.
I didn't say a collision is easy, I said it's obviously false it's harder than colliding a sha256, a space roughly 95780971304118053647396689196894323976171195136475136 times larger.
It /can/ be smaller though, as the probability is linked to the number of global threads generating UUIDs in any given microsecond. Thus if you have enough machines generating UUIDs, you'll have more chances for a collision. Given that it is only possible to generate a collision in a given microsecond, and not globally across ALL TIME, over ∞ time, the probability of a collision(UUID) == 0, while a collision(sha256) == 100%.
