Comment span comes in many forms. That's just one of them. You can do a dozen or so "or's" or you can use preg_match() to look specifically for the URL. Then you can save your or's for even more filters:
// Define a semi-global array so you can update it as you need it. // It's more of a configuration.
"example.com" = your domain name, the reason being, you'll never submit a comment to your self and a common ploy is to use email@example.com in spam.
I will add that wp_die (or any die on error) is not very user friendly (and is lazy) as it forces the user to use their back button. What should happen there is if there's an error, it returns to the form with fields populated.