If it's 64 bit, doesn't that mean you'd need to generate ~10000000000000000000000000000000000000000000000000000000000000000 (2^64) of those numbers to have a collision, not 2^32?
The birthday paradox is named after the non-intuitive fact that with just 32 people in a room you have > 50% of 2 people having a birthday on the same day of the year.
Slight correction: only 23 people, actually. So in every second football ("soccer") game, you have two people on the field with the same birthday.
For example, if you generate 2 numbers and they are the same, but are different to the capnproto number, that’s a collision but doesn’t actually matter.
EDIT: It does apply, I misunderstood what the number was being used for.