If you think “don’t pay off Pascal’s Muggers” is a rule to follow in rule utilitarianism, then you think it’s a rule that maximizes utility, and therefore you’ve already found a reason why paying off Pascal’s Muggers is bad utility before even considering rule utilitarianism. Therefore, I don’t think “it’s a rule in my rule utilitarianism to not do this” is your true rejection to Pascal’s Mugging.
If you think “don’t pay off Pascal’s Muggers” is a rule to follow in rule utilitarianism, then you think it’s a rule that maximizes utility, and therefore you’ve already found a reason why paying off Pascal’s Muggers is bad utility before even considering rule utilitarianism. Therefore, I don’t think “it’s a rule in my rule utilitarianism to not do this” is your true rejection to Pascal’s Mugging.