Comment by GTP

Yes, checking a specific false positive would be trivial and extremely fast[0], as it's just about summing two numbers. But, if the server doesn't do the check, having the site report a counterexample to Goldbach's conjecture would be an easy prank to pull off.

[0] Verifying that those two numbers are actually primes may take more time, but it seems to me that we're still dealing with number sizes for which primality testing isn't that hard. After all, the code here seems to just be using Erathostene's sieve to find primes.