I don't believe so. All websites are scanned regularly.
There's no way to make a website completely hack-proof, so the guarantee means that any security related issues (hacking, malware, etc.) will be fixed at no cost to the customer.
In doing customer development I was surprised to learn how many small businesses had experience with hacking or had security as a primary concern. Website cleanup typically costs a couple hundred dollars on the low-end, so the guarantee provides peace of mind. It's a bit expensive on our end, but it's just an incentive to do an even better job.
Would you call that selling snake-oil? If so, let me know (my contact info is in my profile). I'm not a security expert, but I do hire them and believe the security guarantee is accurate. Please contact me if you disagree or I can improve it, I take this pretty seriously. Thanks for bringing it up, that's why I love posting here.