Even assuming 0 latency on their backend, if you wanted to check each UUIDv4 value again their database during your lifetime, you would need to check 1.686 x 10^27 UUIDv4 per second for 100 years straight. Supercomputers are measured in exaflops, which is 10^18 operations per second, so even distributing the work across many machines, you would need about 1 billion of super computers to be able to have a chance of checking every UUIDv4 value within 100 years.