It does apply, according to https://www.johndcook.com/blog/2017/01/10/probability-of-sec...

You’re right, I misunderstood what the magic number was being used for.

But if my application only uses 100 schemas, I only care about a collision if it's with one of those 100.

You have a collision if any two schemas share the id, not if a specific schema collides with any of the others. So it is exactly like the birthday paradox.

Yeah, but that collision probably doesn’t matter because there’s a bunch of other variables that need to come together for it to be an issue at all.

If the schema id is the message id, in principle it could be an issue as the protocol on the wite would be ambiguous. Then again, you should be able to detect any collisions when you register a schema with the schema repo and deal with it at that time.